N doping of TiO2(110): Photoemission and density-functional studies

Theoretical investigation of CH-bond activation by photocatalytic excited SO2 and the effects of C-, N-, S-, and Se-doped TiO2

The photocatalytic sulfoxidation on TiO₂ discovered by Parrino et al. represents a new, interesting and lower energy route for the synthesis of sulfonic acids. Sulfonic acids are important precursors for dyes, detergents and drugs. In the commonly known industrial process, SO₂ and a specific hydrocarbon are converted into sulfonic acids using high-energy UV light. In this reaction, SO₂ is excited into a metastable triplet state (³SO₂), which has the potential to activate a CH-bond of hydrocarbons and start a radical reaction cycle. By introducing TiO₂ as a photocatalyst, it has been shown that visible light can be used for the synthesis. This offers the potential to be a cost-effective reaction approach for industrial use. However, experimental studies indicate that the initial excitation mechanism of SO₂ on TiO₂ is significantly different from the catalyst-free mechanism. Parrino et al. were able to reveal first evidence for the existence of a charge-transfer process from SO₂ to the TiO₂ surface by means of electrochemical experiments. First theoretical investigations from first principles were able to further substantiate the existence of a charge-transfer. However, to fully understand this mechanism, more accurate methods such as Time Dependent Density Functional Theory (TD-DFT) or ab initio multireference methods such as the Complete Active Space Self Consistent Field (CASSCF) method are required. Furthermore, after understanding the charge-transfer mechanism, the introduction of dopants into TiO₂ can be investigated in order to possibly redshift the excitation energy. This might open the route to using lower energy light for the sulfoxidation of hydrocarbons on TiO₂ as a new potential industrial reaction for the synthesis of sulfonic acids. In this work, we will study the initial step of the photocatalytic sulfoxidation of hydrocarbons using the TD-DFT and CASSCF methods by using a combined approach consisting of calculations with periodic boundary conditions and a newly constructed embedded cluster model. Furthermore, we will explore the effects of doping by introducing four heteroatoms (C, N, S, and Se) into the TiO₂ surfaces anatase[101] and rutile[110] to find a possible enhancement of the photocatalytic reactivity by lowering the electronic excitation energy.

Role of Synthetic Parameters on the Structural and Optical Properties of N,Sn-Copromoted Nanostructured TiO2: A Combined Ti K-Edge and Sn L2,3-Edges X-ray Absorption Investigation

Sn-modification of TiO2 photocatalysts has been recently proposed as a suitable strategy to improve pollutant degradation as well as hydrogen production. In particular, visible light activity could be promoted by doping with Sn2+ species, which are, however, thermally unstable. Co-promotion with N and Sn has been shown to lead to synergistic effects in terms of visible light activity, but the underlying mechanism has, so far, been poorly understood due to the system complexity. Here, the structural, optical, and electronic properties of N,Sn-copromoted, nanostructured TiO2 from sol-gel synthesis were investigated: the Sn/Ti molar content was varied in the 0-20% range and different post-treatments (calcination and low temperature hydrothermal treatment) were adopted in order to promote the sample crystallinity. Depending on the adopted post-treatment, the optical properties present notable differences, which supports a combined role of Sn dopants and N-induced defects in visible light absorption. X-ray absorption spectroscopy at the Ti K-edge and Sn L2,3-edges shed light onto the electronic properties and structure of both Ti and Sn species, evidencing a marked difference at the Sn L2,3-edges between the samples with 20% and 5% Sn/Ti ratio, showing, in the latter case, the presence of tin in a partially reduced state.

Resonant and Selective Excitation of Photocatalytically Active Defect Sites in TiO2

It has been known for several decades that defects are largely responsible for the catalytically active sites on metal and semiconductor surfaces. However, it is difficult to directly probe these active sites because the defects associated with them are often relatively rare with respect to the stoichiometric crystalline surface. In the work presented here, we demonstrate a method to selectively probe defect-mediated photocatalysis through differential alternating current (ac) photocurrent (PC) measurements. In this approach, electrons are photoexcited from the valence band to a relatively narrow distribution of subband gap states in TiO₂ and then subsequently to the ions in solution. Because of their limited number, these defect states fill up quickly, resulting in Pauli blocking, and are thereby undetectable under direct current or continuous wave excitation. In the method demonstrated here, the incident light is modulated with an optical chopper, whereas the PC is measured with a lock-in amplifier. Thin (5 nm) films of TiO₂ deposited by atomic layer deposition on various metal films, including Au, Cu, and Al, exhibit the same wavelength-dependent PC spectra, with a broad peak centered around 2.0 eV corresponding to the band-to-defect transition associated with the hydrogen evolution reaction (HER). While the UV-vis absorption spectra of these films show no features at 2.0 eV, photoluminescence (PL) spectra of these photoelectrodes show a similar wavelength dependence with a peak of around 2.0 eV, corresponding to the subband gap emission associated with these defect sites. As a control, alumina (Al₂O₃) films exhibit no PL or PC over the visible wavelength range. The ac PC plotted as a function of electrode potential shows a peak of around -0.4 to -0.1 V versus normal hydrogen electrode, as the monoenergetic defect states are tuned through a resonance with the HER potential. This approach enables the direct photoexcitation of catalytically active defect sites to be studied selectively without the interference of the continuum interband transitions or the effects of Pauli blocking, which is limited by the slow turnover time of the catalytically active sites, typically on the order of 1 μs. We believe that this general approach provides an important new way to study the role of defects in catalysis in an area where selective spectroscopic studies of these are few.

XANES study of vanadium and nitrogen dopants in photocatalytic TiO2 thin films

X-ray absorption spectroscopy and ab initio simulations unravel the local structure of vanadium and nitrogen dopants in anatase thin films.

Engineering defects and photocatalytic activity of TiO2 nanoparticles by thermal treatments in NH3 and subsequent surface chemical etchings

TiO₂ nanoparticles with N dopants were prepared by thermal treatments in NH₃ and their surface defects were controlled by post chemical etching in HF to find out the influence of the N dopants on photoactivity. The effect of N-doping is found to enhance the photoactivity of TiO₂, but is strongly dependent on the degree of N-doping and the detailed distribution of nitrogen species within the TiO₂ nanoparticles. In particular, the N-rich layers formed near the surface are found to contribute to the enhanced photoactivity due to the reduced band gap. But, the increase in the N concentration may induce defects that act as recombination centers and reduce the photoactivity. Subsequent chemical etching in HF confirms the existence of the substitutional N species near the surface from the observation of paramagnetic N species. But, prolonged HF treatments are found to decrease the photoactivity primarily due to the removal of the N-rich surface layers that are responsible for the enhanced photoactivity. Our results show that the photoactivity of N-doped TiO₂ is strongly influenced by the type and the density of the N dopants induced by the N doping.

The adsorption of SO2 on TiO2 anatase nanoparticles: a density functional theory study

First-principles calculations have been carried out to investigate the adsorption properties of SO2 molecules on nitrogen-doped TiO2 anatase nanoparticles using the density functional theory method to fully exploit the gas-sensing capabilities of TiO2 particles. For this purpose, we have mainly studied the adsorption of the SO2 molecule on the dangling oxygen atom and doped nitrogen atom sites of the TiO2 nanoparticles because these sites are more active than other sites in the adsorption processes. The complex systems consisting of the SO2 molecule positioned toward the undoped and nitrogen-doped nanoparticles have been relaxed geometrically. The results presented include structural parameters such as bond lengths and bond angles and energetics of the systems such as adsorption energies. The electronic structure and its variations resulting from the adsorption process, including the density of states, molecular orbitals, and the charge transfer, are discussed. We found that the adsorption of the SO2 molecule on the nitrogen-doped TiO2 nanoparticles is energetically more favorable than the adsorption on the undoped ones. These results thus provide a theoretical basis for the potential applications of TiO2 nanoparticles in the removal and sensing of SO2 and give an explanation for helping in the optimization of improved gas removers and sensor devices.

Improving the adsorption of sulfur trioxide on TiO2 anatase nanoparticles by N-doping: A DFT study

The adsorptions of sulfur trioxide molecule on undoped and N-doped TiO 2 anatase nanoparticles were investigated by density functional theory (DFT) calculations. N-doped nanoparticles were constructed by substitution of oxygen atoms of TiO2 by nitrogen atoms. The results showed that the adsorption energies of SO3 on the different nanoparticles following the order N-doped (N site)>N-doped ( O D site)>Undoped ( O D site). We provide the electronic structure of the nanoparticles, as well as complex systems containing the sulfur trioxide molecule and discuss the key issues that influence the adsorption process. The structural properties including the bond lengths, bond angles and adsorption energies and the electronic properties including the projected density of states (PDOSs) and molecular orbitals (MOs) have been mainly analyzed in detail. The obtained results indicate that the interaction between SO 3 molecule and N-doped TiO 2 nanoparticle is stronger than that between SO 3 and undoped nanoparticle, which suggests that N-doping helps to strengthen the interaction of SO 3 with TiO 2 anatase nanoparticles. It is shown that although SO 3 molecule has no significant interaction with undoped nanoparticle, it tends to be strongly adsorbed to N-doped anatase nanoparticles with considerable adsorption energies, being as an effective property to be utilized in gas sensing applications. We also note at this point that the titanium atom and the doped nitrogen atom sites are more active than the dangling oxygen site, which reveals that the titanium and doped nitrogen sites provide more stable adsorption geometries.

Molecular nitrogen in N-doped TiO2 nanoribbons

X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure (NEXAFS) support the idea that during nitrogen doping of TiO₂ nanoribbons N₂ molecules may be formed and trapped in the nanostructures.

Self-organized vanadium and nitrogen co-doped titania nanotube arrays with enhanced photocatalytic reduction of CO2 into CH4

Self-organized V-N co-doped TiO2 nanotube arrays (TNAs) with various doping amount were synthesized by anodizing in association with hydrothermal treatment. Impacts of V-N co-doping on the morphologies, phase structures, and photoelectrochemical properties of the TNAs films were thoroughly investigated. The co-doped TiO2 photocatalysts show remarkably enhanced photocatalytic activity for the CO2 photoreduction to methane under ultraviolet illumination. The mechanism of the enhanced photocatalytic activity is discussed in detail.

Promotional effect of silver nanoparticles on the performance of N-doped TiO2 photoanode-based dye-sensitized solar cells

We report the first successful application of an N-TiO₂–Ag nanocomposite as an efficient photoanode for highly efficient dye-sensitized solar cells (DSSC).

Synergistic modification of electronic and photocatalytic properties of TiO2 nanotubes by implantation of Au and N atoms

The structural and electronic properties of N-doped, Au-adsorbed, and Au/N co-implanted TiO2 nanotubes (NTs) were investigated by performing first-principle density functional theory (DFT) calculations. For all the possible implanted configurations, the radius and bond length do not change significantly relative to the clean NTs. Our results indicate that the introduction of N into NTs is in favor of implantation of Au, and Au pre-adsorption on the NTs can also enhance the N concentration in NTs. The synergistic stability can be mainly attributed to charge transfer between Au and N atoms. In co-implanted configurations, the empty N 2p states in the band gap are occupied by one electron; denoted by Au 5s states. Thus, the associated electron transition among the valence band, the conduction band and the gap states results in redshift of the light absorption. In addition, the disappearance of N 2p empty states can effectively decrease the photogenerated carrier combination. Therefore, the Au/N implanted NTs should be regarded as a promising photocatalytic material under the visible light region.

CARBON-DOPEDTiO2NANOTUBES: EXPERIMENTAL AND COMPUTATIONAL STUDIES

C-doped TiO2nanotubes (NTs) with anatase structure, prepared by anodizing the polished Ti foils, were characterized using X-ray diffraction (XRD), field-emission scanning electron microscope (FE-SEM), and synchrotron-based X-ray photoemission spectroscopy (XPS). XPS results show electron losses in C atoms, no electron change in Ti atoms, and two doping energy levels appeared in band gaps. Structural geometries, DOSs, PDOSs, and Bader charge analyses of C -doped TiO2anatase are predicted by periodic DFT calculations. Eight doping positions were taken into consideration: two substitutional cases (in oxygen and titanium sites) and six interstitial cases. We found that the interstitial carbon doping type is the most stable one, whereas the substitutional cases are rather unstable. Band-gap modifications can also be found in oxygen substitution, but not in titanium substitution. Both band-gap modification and non-band-gap modification are found in the interstitial carbon doping. In these eight C -doping systems, only the C atom in the oxygen substitution case gains electrons, 1.14 e, and others present electron losses within 0.5–4.00 e. The results of XPS measurements, DOSs calculations, and Bader charge analyses show that carbon interstitial is the most likely doping type for the C -doped TiO2NTs.

Spectroscopic assessment of the role of hydrogen in surface defects, in the electronic structure and transport properties of TiO2, ZnO and SnO2 nanoparticles

The interaction of metal oxides with gases is very important for the operation of energy devices such as fuel cells and gas sensors, and also relevant for materials synthesis and processing. The electronic transport properties of metal oxides for the aforementioned devices strongly depend on the chemistry of these gases and on the presence or absence of defects on the surface and in the bulk. The Debye screening length is in this respect a material specific property which becomes particularly significant when the material is comprised of nanoparticles. In the present study, poly-crystalline TiO(2), ZnO and SnO(2) nanoparticles were synthesized by a high temperature flame spray combustion process and investigated for their interaction with hydrogen. The chemistry of the reduced and oxidized surfaces of these metal oxides, where the interaction with gases takes place, was investigated in detail with X-ray photoelectron spectroscopy (XPS). The transitions found near E(F) with XPS are consistent with those found by diffuse reflectance spectroscopy (DRS) and were assigned to surface states originating from non-equilibrium oxygen vacancies. We show how the non-stoichiometric character of the metal oxide surface constitutes electronic surface defects, in particular oxygen vacancies which allow for additional transitions near the Fermi energy (E(F)). The concentration of these surface defects can be strongly reduced by thermal after-treatment under air or increased by Ar(+)-sputtering, after which significant spectral features appear near E(F). Such prominent changes are particularly observed for TiO(2) and SnO(2), whereas the stoichiometry of the ZnO surface seems to be less responsive to such reducing and oxidizing conditions. Pronounced changes of the electrical conductivity upon changing from reducing to oxidizing conditions at elevated temperatures were monitored by electrochemical impedance spectroscopy (EIS). The lowering of the potential barrier for the charge transport particularly at lower temperatures already at reducing conditions is confirmed. The impedance response indicates that charge transfer is governed predominantly by one single process, i.e. by interaction of surface-like states localized within depletion layer with gas molecules. This implies that the free charge carriers in the material are determined by the intrinsic property like non-stoichiometry. Gas sensors made from such FSS nanoparticulate metal oxides showed well developed sensing characteristics of the hydrogen sensing at moderate temperatures.

Photoelectrochemical study of oxygen deficient TiO2 nanowire arrays with CdS quantum dot sensitization

Oxygen-deficient TiO(2) nanowires show substantially increased donor density due to an increase in oxygen vacancies introduced intentionally by thermal treatment in ammonia, vacuum, or hydrogen. By coupling oxygen-deficient TiO(2) nanowires with CdS quantum dots, a significant enhancement in their photoactivities was observed in the entire wavelength region from 350 to 550 nm. These new nanocomposites hold great promise for solar hydrogen generation.

Synergistic effect of CdSe quantum dot sensitization and nitrogen doping of TiO(2) nanostructures for photoelectrochemical solar hydrogen generation

We report the synthesis and photoelectrochemical (PEC) studies of TiO(2) nanoparticles and nanowires simultaneously doped with nitrogen and sensitized with CdSe quantum dots (QDs). These novel nanocomposite structures have been applied successfully as photoanodes for PEC hydrogen generation using Na(2)S and Na(2)SO(3) as sacrificial reagents. We observe significant enhanced photoresponse in these nanocomposites compared to N-doped TiO(2) or CdSe QD sensitized TiO(2). The enhancement is attributed to the synergistic effect of CdSe sensitization and N-doping that facilitate hole transfer/transport from CdSe to TiO(2) through oxygen vacancy states (V(o)) mediated by N-doping. The results demonstrate the importance of designing and manipulating the energy band alignment in composite nanomaterials for fundamentally improving charge separation and transport and thereby PEC properties.