N Doping of Rutile TiO2 (110) Surface. A Theoretical DFT Study (original) (raw)
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N incorporation and electronic structure in N-doped TiO2(110) rutile
Surface Science, 2007
We describe the growth and properties of well-defined epitaxial TiO 2Àx N x rutile for the first time. A mixed beam of atomic N and O radicals was prepared in an electron cyclotron resonance plasma source and Ti was supplied from a high-temperature effusion cell or an electron beam evaporator, depending on the required flux. A very high degree of structural quality is generally observed for films grown under optimized anion-rich conditions. N substitutes for O in the lattice, but only at the $1 at.% level, and is present as N 3À . Epitaxial growth of TiO 2 and TiO 2Àx N x rutile prepared under anion-rich conditions is accompanied by Ti indiffusion, leading to interstitial Ti (Ti i ), which is a shallow donor in rutile. Our data strongly suggest that Ti i donor electrons compensate holes associated with substitutional N 2À (i.e., Ti(III) + N 2À ! Ti(IV) + N 3À ), leading to highly resistive or weakly n-type, but not p-type material. Ti 2p core-level line shape analysis reveals hybridization of N and Ti, as expected for substitutional N. Ti-N hybridized states fall in the gap just above the VBM, and extend the optical absorption well into the visible.
Carbon Doping of the TiO 2 (110) Rutile Surface. A Theoretical Study Based on DFT
Chemistry of Materials, 2009
ABSTRACT A detailed analysis of the structural and electronic properties of the C-doped rutile TiO2(110) surface has been performed by means of periodic density-functional calculations. C atoms adsorb exothermically on the surface, although they are unstable with respect to the CO escape and formation of an oxygen vacancy. C implantation at lattice positions is an endothermic process, in contrast with what was observed in the case of N implantation. These C implanted atoms are also unstable in the presence of molecular oxygen. A strong cooperative interaction between implanted C atoms and surface oxygen vacancies is observed: (i) the presence of vacancies significantly lowers the implantation energy and stabilizes the C atoms, although not enough to avoid their escape in the presence of O2; (ii) the presence of implanted C atoms noticeably lowers the energy of formation of oxygen vacancy in the surface. Finally, an analysis of the electronic structure confirms the presence of in-gap states that could improve the photocatalytic activity of rutile upon being doped with C.
Journal of Physical Chemistry C, 2009
N-doped TiO 2 thin films have been prepared by plasma enhanced chemical vapor deposition and by physical vapor deposition by adding nitrogen or ammonia to the gas phase. Different sets of N-doped TiO 2 thin films have been obtained by changing the preparation conditions during the deposition. The samples have been characterized by X-ray diffraction, Raman, UV-vis spectroscopy, and X-ray photoemission spectroscopy (XPS). By changing the preparation conditions, different structures, microstructures, and degrees and types of doping have been obtained and some relationships have been established between these film properties and their visible light photoactivity. The N1s XP spectra of the samples are characterized by three main features, one tentatively attributed to TiN (i.e., nitride with a binding energy (BE) of 396.1 eV) and two others with BEs of 399.3 and 400.7 eV, tentatively attributed to nitrogen bonded simultaneously to titanium and oxygen atoms (i.e., TiN -O like species). By controlling the deposition conditions it is possible to prepare samples with only one of these species as majority component. It has been shown that only the samples with TiN -O like species show surface photoactivity being able to change their wetting angle when they are illuminated with visible light. The presence of these species and an additional complex structure formed by a mixture of anatase and rutile phases is an additional condition that is fulfilled by the thin films that also present photocatalytic activity with visible light (i.e., surface and Schottky barrier driven photoactivities). The relationships existing between the reduction state of the samples and the formation of TiN or TiN -O like species are also discussed.
First-Principles Study of S Doping at the Rutile TiO 2 (110) Surface
The Journal of Physical Chemistry C, 2009
The structural, energetic and electronic properties of various S doping configurations by substitution and adsorption at the rutile TiO 2 (110) surface have been investigated by first-principles density functional theory (DFT) calculations. The stability of these configurations has been compared on the basis of the calculated formation and adsorption energies. Our results indicate that S dopants replace surface O atoms or bind to Ti atoms preferentially. Moreover, implantation of S dopants into the rutile lattice favored the formation of oxygen vacancies, which promotes further S incorporation. Doping of single S atoms into Ti sites (S-cation doping) led to relatively small reductions of the photon transition energy, while S-substitution of O atoms (S-anion doping) and adsorption on the surface (S-cation/anion doping)
Journal of Physical Chemistry C, 2009
N-doped TiO 2 thin films have been prepared by plasma enhanced chemical vapor deposition and by physical vapor deposition by adding nitrogen or ammonia to the gas phase. Different sets of N-doped TiO 2 thin films have been obtained by changing the preparation conditions during the deposition. The samples have been characterized by X-ray diffraction, Raman, UV-vis spectroscopy, and X-ray photoemission spectroscopy (XPS). By changing the preparation conditions, different structures, microstructures, and degrees and types of doping have been obtained and some relationships have been established between these film properties and their visible light photoactivity. The N1s XP spectra of the samples are characterized by three main features, one tentatively attributed to Ti-N (i.e., nitride with a binding energy (BE) of 396.1 eV) and two others with BEs of 399.3 and 400.7 eV, tentatively attributed to nitrogen bonded simultaneously to titanium and oxygen atoms (i.e., Ti-N-O like species). By controlling the deposition conditions it is possible to prepare samples with only one of these species as majority component. It has been shown that only the samples with Ti-N-O like species show surface photoactivity being able to change their wetting angle when they are illuminated with visible light. The presence of these species and an additional complex structure formed by a mixture of anatase and rutile phases is an additional condition that is fulfilled by the thin films that also present photocatalytic activity with visible light (i.e., surface and Schottky barrier driven photoactivities). The relationships existing between the reduction state of the samples and the formation of Ti-N or Ti-N-O like species are also discussed.
X-ray emission spectroscopy (XES) and X-ray absorption spectroscopy (XAS) provide a unique opportunity to probe both the highest occupied and the lowest unoccupied states in matter with bulk sensitivity. In this work, a combination of valence-to-core XES and pre-edge XAS techniques are used to determine changes induced in the electronic structure of titanium dioxide doped with nitrogen atoms. Based on the experimental data it is shown that N-doping leads to incorporation of the p-states on the occupied electronic site. For the conduction band, a decrease in population of the lowest unoccupied d-localized orbitals with respect to the d-delocalized orbitals is observed. As confirmed by theoretical calculations, the N p-states in TiO 2 structure are characterized by higher binding energy than the O p-states which gives a smaller value of the band-gap energy for the doped material.
Journal of Physical Therapy Science
The non-metals doping as an anion to alternate the electronic structure and optical absorption property of TiO2, and ultimately, to induce the visible light activity for it have been the purpose of this work. The electronic structure and optical properties of neon-doped rutile TiO2 have been investigated by using the density functional theory with Slater type orbital basis set and correlation. This was done using the PBE96 method as implemented within the HyperChem 7.52 software package with Ne concentration approaching the low level may present in industrial samples of rutile TiO2. Defect states involving substitution of an oxygen atom for a neon atom were studied along with the more stable configuration of one neon substitution. Neon changed the band structure and led to a reduce in the band gap in rutile. This means that neon doping brings the absorption edge into the visible range and therefore increase the photocatalytic activity. A stronger absorption for anionic Ne-doped TiO2...
Applied Catalysis B-environmental, 2010
Complex N (NO)-doped TiOx films on the glass substrate were prepared by radio-frequency (RF) magnetron reactive sputtering of Ti target in a mixed gas of argon and dry air with low oxygen concentrations. The surface doping states and energy band gap properties were studied by XPS and density-functional theory (DFT) applied to a 2 × 2 × 1 supercell of N-doped TiO2 with oxygen deficiency. Although all the films exhibit an anatase structure, the photocatalytic properties as well as other film properties (lattice parameters, grain sizes, introduced nitrogen contents, and optical properties) largely depend on the air flow ratios. The electronic bonding configurations at the nanoscale film surface are extremely important for photocatalysis, and there appears an optimal surface nitrogen amount incorporated in the anatase TiO2 lattice. Reduced Ti ions (and regions) at the nanoscale surface are proposed to play an important role by providing a local charge imbalance through the Schottky-barrier-like mechanism. Our DFT calculation shows the modified band calculation, especially for the film surface, involving both oxygen deficiency and N (NO) doping is important, due to the variation in the number and location of the impurity levels in the energy band gap. It is suggested that interstitial NOx (or substitutional NO) doping states with oxygen vacancy involving N–Ti–O or Ti–N–O bondings (linkages) are more effective on photocatalysis than the substitutional N doping states with oxygen vacancy. The seemingly desired impurity energy level(s) introduced in the electronic band gap does not necessarily improve photocatalysis, despite the desired optical properties observed, due to the active recombination sites newly produced and the complexity of the nanoscale surface science.
Surface and Electronic Structure of Titanium Dioxide Photocatalysts
The Journal of Physical Chemistry B, 2000
TiO 2 films prepared by sol-gel route are active photocatalysts for the oxidation of organics in photoelectrochemical cells. The as-grown films for photocatalysis applications and those exposed to Ar + or H 2 + +Ar + ion bombardment are characterized by different spectroscopic methods, such as X-ray diffraction (XRD), atomic force microscopy (AFM), UV-vis transmittance, photothermal deflection spectroscopy (PDS) and X-ray photoelectron spectroscopy (XPS), as well as by conductance. This material has defects associated with oxygen vacancies produced during the sample preparation which support nondissociative adsorption of O 2 when films are exposed to air. Charge transfer from reduced Ti species to adsorbed dioxygen leads to Ti-O 2surface complexes that are partially removed by heating at 200°C, and fully removed after 30 min ion bombardment. By comparison with the relatively well-understood structural defects of bombarded TiO 2 we arise to a quite complete structural model of the as grown material which corresponds to an amorphous semiconductor possessing relative low disorder and density of states as compared with a pure amorphous material. These TiO 2 films are modeled as low size crystalline domain embedded in an amorphous matrix whose electronic structure exhibit exponential band tails and a narrow band close to the conduction band. The latter is fully or partially occupied depending on the presence of adsorbed electron scavengers such as dioxygen.