Titanium(IV) complexes as direct TiO2 photosensitizers (original) (raw)
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Journal of the American Chemical Society, 2014
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The Journal of Physical Chemistry C, 2012
Photoselective oxidation yielding high-added value chemicals appears as a green novel process with potential to be explored. In this study we combine spectroscopic XPS (N 1s and O 1s) and multiwavelength Raman data with density functional theory calculations to explore the structural and electronic properties of W,N-codoped TiO 2 anatase surfaces and interpret the outstanding photocatalytic properties of such a system in partial oxidation reactions. Theoretical calculations allow us to examine several substitutional and N-interstitial configurations at different concentrations of the W,N dopants (similar to those experimentally found), as well as their interaction with structural point defects: Ti cation vacant sites and surface wolframyl species that are required to compensate the extra charge of the W 6+ and N-containing anions. The joint use of theoretical and experimental XPS and Raman tools renders key structural information of W,N-codoped microcrystalline TiO 2 solids. The incorporation of N at substitutional positions of anatase with the concomitant presence of WO species introduces localized states in the band gap, a result that is critical in interpreting the chemical behavior of the solids. The combination of the electronic and geometric structural information leads to a simple mechanism that rationalizes the experimentally observed photoactivity and selectivity in partial oxidation reactions.
Homogeneous Photosensitization of Complex TiO2 Nanostructures for Efficient Solar Energy Conversion
Scientific Reports, 2012
TiO 2 nanostructures-based photoelectrochemical (PEC) cells are under worldwide attentions as the method to generate clean energy. For these devices, narrow-bandgap semiconductor photosensitizers such as CdS and CdSe are commonly used to couple with TiO 2 in order to harvest the visible sunlight and to enhance the conversion efficiency. Conventional methods for depositing the photosensitizers on TiO 2 such as dip coating, electrochemical deposition and chemical-vapor-deposition suffer from poor control in thickness and uniformity, and correspond to low photocurrent levels. Here we demonstrate a new method based on atomic layer deposition and ion exchange reaction (ALDIER) to achieve a highly controllable and homogeneous coating of sensitizer particles on arbitrary TiO 2 substrates. PEC tests made to CdSe-sensitized TiO 2 inverse opal photoanodes result in a drastically improved photocurrent level, up to ,15.7 mA/cm 2 at zero bias (vs Ag/AgCl), more than double that by conventional techniques such as successive ionic layer adsorption and reaction. E ver since the seminal paper of photoelectrolysis of water by Fujishima and Honda 1 , TiO 2 has received wide attentions in photocatalysts, water splitting and solar cells due to its high photoactivity, low cost and excellent chemical stability 2-5. The limiting factor for TiO 2 is the large band gap (,3.2 eV), which defines its light absorption only in the UV range. During the past three decades, tremendous efforts have been put to enhance the visible light harvesting ability of TiO 2 6. Heterogeneous structures have been proposed to couple TiO 2 with materials exhibiting visible light harvesting ability, and the first trial was done by Serpone et al. to couple TiO 2 with CdS which showed a significant improvement 7. Later on Graetzel made a significant breakthrough in sensitizing TiO 2 with dye molecules, viz., the dye-sensitized TiO 2 photoanode 4. Following the invention of Graetzel cell, quantum dot sensitized solar cells (QDSSC) quickly catch up following the mature quantum dot synthesis protocol developed by Peng and Alivisatos 8,9. The key development of QDSSC was made by Kamat in 2005, with the pre-synthesized CdSe nanocrystals linked to TiO 2 thin films by organic molecules 10. Since then various methods of sensitization have been developed, and they can be summarized into two main categories: assembly of pre-synthesized QDs and direct growth 11,12. Pre-synthesis provides the feasibility of facile control in the size, size distribution and morphology. However, the charge transfer would be retarded by the surface functional molecules. Also the loading of the sensitizer prepared by this method is usually low. Direct growth allows both a compact contact of the sensitizer with TiO 2 , and the ease of increasing the loading of the sensitizer. A diverse range of methods are reported for the direct coating of the sensitizing materials, such as chemical bath deposition 13,14 , successive ionic layer adsorption and reaction (SILAR) 15-17 , electrochemical deposition 18 , chemical vapour deposition 19 and electrophoretic deposition 20. Despite the development of various sensitization methods, the sensitizers still suffer from poor thickness and uniformity control especially for deposition on high aspect-ratio TiO 2 nanostructures. As the size of QDs is much larger than dye molecules, penetration of QDs into TiO 2 nanoarchitectures with a depth .10 mm is more difficult than the case in dye-sensitized solar cells 11. Due to the quantum confinement effect and the limited charge diffusion length, the size of the QDs plays an important role in charge transfer process. The poor control in conventional deposition techniques usually leads to aggregation of QDs into large particles, thus causing high internal recombination loss. Atomic layer deposition (ALD) is a thin film deposition technique that is based on self-limiting surface reactions by sequential exposure of the substrate to different gas phase precursors 21. ALD provides precise thickness control at the angstrom or monolayer level and deposition on high aspect ratio nanostructures with
From Ultrafast Processes and Nanostructures to Optoelectronics, Energy Storage and Nanomedicine, 2011
Titanium dioxide is one of the most widely investigated oxides. This is due to its broad range of applications, from catalysis to photocatalysis to photovoltaics. Despite this large interest, many of its bulk properties have been sparsely investigated using either experimental techniques or ab initio theory. Further, some of TiO 2 's most important properties, such as its electronic band gap, the localized character of excitons, and the localized nature of states induced by oxygen vacancies, are still under debate. We present a unified description of the properties of rutile and anatase phases, obtained from ab initio state of the art methods, ranging from density functional theory (DFT) to many body perturbation theory (MBPT) derived techniques. In so doing, we show how advanced computational techniques can be used to quantitatively describe the structural, electronic, and optical properties of TiO 2 nanostructures, an area of fundamental importance in applied research. Indeed, we address one of the main challenges to TiO 2 -photocatalysis, namely band gap narrowing, by showing how to combine nanostructural changes with doping. With this aim we compare TiO 2 's electronic properties for 0D clusters, 1D nanorods, 2D layers, and 3D bulks using different approximations within DFT and MBPT calculations. While quantum confinement effects lead to a widening of the energy gap, it has been shown that substitutional doping with boron or nitrogen gives rise to (meta-)stable structures and the introduction of dopant and mid-gap states which effectively reduce the band gap. Finally, we report how ab initio methods can be applied to understand the important role of TiO 2 as electron-acceptor in dye-sensitized solar cells. This task is made more difficult by the hybrid organic-oxide structure of the involved systems.
Physical Review Letters, 2009
Noncompensated n-p codoping'' is established as an enabling concept for enhancing the visible-light photoactivity of TiO 2 by narrowing its band gap. The concept embodies two crucial ingredients: The electrostatic attraction within the n-p dopant pair enhances both the thermodynamic and kinetic solubilities, and the noncompensated nature ensures the creation of tunable intermediate bands that effectively narrow the band gap. The concept is demonstrated using first-principles calculations, and is validated by direct measurements of band gap narrowing using scanning tunneling spectroscopy, dramatically redshifted optical absorbance, and enhanced photoactivity manifested by efficient electron-hole separation in the visible-light region. This concept is broadly applicable to the synthesis of other advanced functional materials that demand optimal dopant control.
Tuning the optical and photoelectrochemical properties of surface-modified TiO2
Photochemical & Photobiological Sciences, 2008
Surface-modification of TiO 2 is found to be a powerful tool for manipulating the fundamental optical and photoelectrochemical properties of TiO 2. High surface area nanocrystalline TiO 2 was modified by urea pyrolysis products at different temperatures between 300 • C and 500 • C. Modification occurs through incorporation of nitrogen species containing carbon into the surface structure of titania. The N1s XPS binding energies are 399-400 eV and decrease with increasing modification temperature whereby the Ti2p 3/2 peak is also shifted to lower binding energies by about 0.5 eV. With increasing modification temperature the optical bandgap of surface-modified TiO 2 continuously decreases down to ∼2.1 eV and the quasi-Fermi level of electrons at pH 7 is gradually shifted from −0.6 V to −0.3 V vs. NHE. The surface-modified materials show enhanced sub-bandgap absorption (Urbach tail) and exhibit photocurrents in the visible down to 750 nm. The maximum incident photon-to-current efficiency (IPCE) was observed for the materials modified at 350 • C and 400 • C (IPCE ∼ 14% at 400 nm, and IPCE ∼ 1% at 550 nm, respectively). The efficiency of photocurrent generation is limited by surface recombination, which leads to a significant decrease in IPCE values and significantly changes the shape of the IPCE spectra in dependence on the optical bandgap.