EPR investigation of TiO2 nanoparticles with temperature-dependent properties

An in situ electron paramagnetic resonance (EPR) study has been carried out for anatase (Hombikat UV100) and rutile TiO(2) nanoparticles at liquid helium (He) temperature (4.2 K) under UV irradiation. Rutile titania was synthesized by ultrasonic irradiation with titanium tetrachloride (TiCl(4)) as the precursor. XRD and Raman results evidence the crystallinity of titania phases. The nature of trapped electrons and holes has been investigated by EPR spectroscopy under air and vacuum conditions. Illumination of TiO(2) powder (anatase and rutile) at 4.2 K resulted in the detection of electrons being trapped at Ti(4+) sites within the bulk and holes trapped at lattice oxide ions at the surface. The stability of electron traps was very sensitive to temperature in both phases of TiO(2). The annealing kinetics of the EPR detected radicals has been studied from 4.2 K to ambient temperature and also for calcined titania particles from 523 to 1273 K.     

Complex and charge transfer between TiO2 and pyrroloquinoline quinone

Pyrroloquinoline quinone (PQQ) forms a tridentate complex with coordinatively unsaturated titanium atoms on the surface of approximately 4.5 nm TiO2 particles; an association constant of K = 550 M-1 per Ti(IV)surf has been determined. Low-temperature electron paramagnetic resonance was employed in identification of localized charges and consequently produced radicals and in determination of charge-transfer processes. The photoexcitation of the PQQ-TiO2 complex results in the transfer of conduction band electrons from TiO2 to bound PQQ and the formation of the semiquinone radical. Attaching dopamine (DA) as an electron donor and PQQ as an electron acceptor on the surface of TiO2 results in spatial separation of photogenerated charges; the holes localize on dopamine and electrons on PQQ, with higher yields than for each component separately. In this triad-type assembly (PQQ-TiO2/DA) the PQQ that is bound to the particles acts as a sink for electrons allowing their almost complete scavenging even at temperature as low as 4 K.     

Charge trapping and photoadsorption of O2 on dehydroxylated TiO2 nanocrystals--an electron paramagnetic resonance study.

The interaction of photogenerated charges with molecular oxygen was investigated on TiO2 nanocrystals by means of paramagnetic resonance (EPR) spectroscopy. Compared to photoactivation experiments in vacuum at P < 10(-6) mbar and T = 140 K, the presence of O2 enhances the concentration of persistently trapped electron and hole centres--by a factor of ten--due to the formation of adsorbed O2- species. The photoadsorption of oxygen was also tracked quantitatively by pressure measurements, and the number of trapped charges, hole centres and O2- was found to correspond to ten electron-hole pairs per TiO2 nanocrystal. Conversely, in experiments at P < 10(-6) mbar with one trapped electron-hole pair per particle, charge separation is not persistent and completely reversible with respect to temperature. Heating to 298 K causes the total annihilation of photogenerated and trapped charges.     

Low temperature kinetics and energetics of the electron and hole traps in irradiated TiO2 nanoparticles as revealed by EPR spectroscopy

We have monitored exclusively the dynamics of photogenerated charge carriers trapping in deep traps and trapped electron-hole recombination in UV irradiated anatase TiO2 powders by electron paramagnetic resonance (EPR) spectroscopy at 10 K. The results reveal that the strategy of using low temperatures contributes to the stabilization of the charged pair states for hours by reducing the rate of electron-hole recombination processes. Since only the localized states such as holes trapped at oxygen anions and electrons trapped at coordinatively unsaturated cations are accessible to EPR spectroscopy, the time-dependent population and depopulation of these EPR signals reflect the kinetics and energetics of these trap states. The data support a model of sequential accumulation of deep trap site populations in which the initial fast direct trapping into a deep trap site is followed by slower carrier trap-to-trap hopping until a deep trap is encountered for both photogenerated electrons and holes. Effective modeling of the subsequent decay of trapped-holes is achieved by employing a first-order kinetics, whereas the decay of either surface- or inner-trapped electrons has both a fast and a slow component. The fast component is attributed to a trapped-electron and a free-hole recombination, and the slow component is attributed to trapped electron-hole recombination. The activation energies for the process of diffusion of trapped electrons from their Ti3+ trapping sites are estimated.     

An EPR study of two-electron sensitization by fragmentable electron donors

Fragmentable electron donors (FEDs) are molecules designed to undergo bond fragmentation after capturing the hole created by photoexposure of silver halide. By design, the radical remaining after fragmentation is a potent reductant expected to be capable of injecting an electron into the conduction band of the silver halide. Thus, the addition of a FED to the AgX surface should allow the creation of two electrons for each photon absorbed by the substrate. Photographic studies have confirmed that the addition of FEDs can increase the photosensitivity of AgX materials. In this work, EPR spectroscopy was employed to study the processes of hole capture, donor fragmentation, and secondary electron injection by FEDs in AgBr dispersions. To do so, we used AgBr microcrystals doped with diamagnetic transition metal complexes that act as deep electron traps. For samples exposed to actinic light at 15 K, secondary electron injection was detected as an increase in the EPR signal from electrons trapped at the dopant upon annealing the samples above 50 K. Organic radical intermediates and self-trapped hole centers were the other paramagnetic species monitored in this study. The results presented here confirm that the FED sensitization mechanisms originally proposed by Gould et al. take place at silver halide surfaces and result in additional electrons in the silver halide conduction band.     

Gold nanoparticles located at the interface of anatase/rutile TiO2 particles as active plasmonic photocatalysts for aerobic oxidation

Visible-light irradiation (λ > 450 nm) of gold nanoparticles loaded on a mixture of anatase/rutile TiO(2) particles (Degussa, P25) promotes efficient aerobic oxidation at room temperature. The photocatalytic activity critically depends on the catalyst architecture: Au particles with <5 nm diameter located at the interface of anatase/rutile TiO(2) particles behave as the active sites for reaction. This photocatalysis is promoted via plasmon activation of the Au particles by visible light followed by consecutive electron transfer in the Au/rutile/anatase contact site. The activated Au particles transfer their conduction electrons to rutile and then to adjacent anatase TiO(2). This catalyzes the oxidation of substrates by the positively charged Au particles along with reduction of O(2) by the conduction band electrons on the surface of anatase TiO(2). This plasmonic photocatalysis is successfully promoted by sunlight exposure and enables efficient and selective aerobic oxidation of alcohols at ambient temperature.     

Density functional study of the interfacial electron transfer pathway for monolayer-adsorbed InN on the TiO(2) anatase (101) surface

Density functional theory (DFT) in connection with ultrasoft pseudopotential (USP) and generalized gradient spin-polarized approximations (GGSA) is applied to calculate the adsorption energies and structures of monolayer-adsorbed InN on the TiO(2) anatase (101) surface and the corresponding electronic properties, that is, partial density of states (PDOS) for surface and bulk layers of the TiO(2) anatase (101) surface and monolayer-adsorbed InN, to shed light on the possible structural modes for initial photoexcitation within the UV/vis adsorption region followed by fast electron injection through the InN/TiO(2) interface for an InN/TiO(2)-based solar cell design. Our calculated adsorption energies found that the two most probable stable structural modes of monolayer-adsorbed InN on the TiO(2) anatase (101) surface are (1) an end-on structure with an adsorption energy of 2.52 eV through N binding to surface 2-fold coordinated O (O(cn2)), that is, InN-O(cn2), and (2) a side-on structure with an adsorption energy of 3.05 eV through both N binding to surface 5-fold coordinated Ti (Ti(cn5)) and In bridging two surface O(cn2), that is, (O(cn2))(2)-InN-Ti(cn5). Our calculated band gaps for both InN-O(cn2) and (O(cn2))2-InN-Ti(cn5) (including a 1.0-eV correction using a scissor operator) of monolayer-adsorbed InN on the TiO(2) anatase (101) surface are red-shifted to 1.7 eV (730 nm) and 2.3 eV (540 nm), respectively, which are within the UV/vis adsorption region similar to Gratzel's black dye solar cell. Our analyses of calculated PDOS for both surface and bulk layers of the TiO(2) anatase (101) surface and monolayer-adsorbed InN on the TiO(2) anatase (101) surface suggest that the (O(cn2))(2)-InN-Ti(n5) configuration of monolayer-adsorbed InN on the TiO(2) anatase (101) surface would provide a more feasible structural mode for the electron injection through the InN/TiO(2) interface. This is due to the presence of both occupied and unoccupied electronic states for monolayer-adsorbed InN within the band gap TiO(2) anatase (101) surface, which will allow the photoexcitation within the UV/vis adsorption region to take place effectively, and subsequently the photoexcited electronic states will overlap with the unoccupied electronic states around the lowest conduction band of the TiO(2) anatase (101) surface, which will ensure the electron injection through the InN/TiO(2) interface. Finally, another thing worth our attention is our preliminary study of double-layer-adsorbed InN on the TiO(2) anatase (101) surface, that is, (O(cn2))(2)-(InN)(2)-Ti(cn5), with a calculated band gap red-shifted to 2.6 eV (477 nm) and a different overlap of electronic states between double-layer-adsorbed InN and the TiO(2) anatase (101) surface qualitatively indicated that there is an effect of the thickness of adsorbed InN on the TiO(2) anatase (101) surface on both photoexcitation and electron injection processes involved in the photoinduced interfacial electron transfer through InN/TiO(2). A more thorough and comprehensive study of different layers of InN adsorbed in all possible different orientations on the TiO(2) anatase (101) surface is presently in progress.     

Probing defect sites on TiO2 with [Re3(CO)12H3]: spectroscopic characterization of the surface species

Samples of the anatase phase of titania were treated under vacuum to create Ti(3+) surface-defect sites and surface O(-) and O(2) (-) species (indicated by electron paramagnetic resonance (EPR) spectra), accompanied by the disappearance of bridging surface OH groups and the formation of terminal Ti(3+)-OH groups (indicated by IR spectra). EPR spectra showed that the probe molecule [Re(3)(CO)(12)H(3)] reacted preferentially with the Ti(3+) sites, forming Ti(4+) sites with OH groups as the [Re(3)(CO)(12)H(3)] was adsorbed. Extended X-ray absorption fine structure (EXAFS) spectra showed that these clusters were deprotonated upon adsorption, with the triangular metal frame remaining intact; EPR spectra demonstrated the simultaneous removal of surface O(-) and O(2) (-) species. The data determined by the three complementary techniques form the basis of a schematic representation of the surface chemistry. According to this picture, during evacuation at 773 K, defect sites are formed on hydroxylated titania as a bridging OH group is removed, forming two neighboring Ti(3+) sites, or, when a Ti(4+)-O bond is cleaved, forming a Ti(3+) site and an O(-) species, with the Ti(4+)-OH group being converted into a Ti(3+)-OH group. When the probe molecule [Re(3)(CO)(12)H(3)] is adsorbed on a titania surface with Ti(3+) defect sites, it reacts preferentially with these sites, becoming deprotonated, removing most of the oxygen radicals, and healing the defect sites.     

The different behavior of rutile and anatase nanoparticles in forming oxy radicals upon illumination with visible light: an EPR study

Photoexcited TiO(2) has been found to generate reactive oxygen species, yet the precise mechanism and chemical nature of the generated oxy species especially regarding the different crystal phases remain to be elucidated. Visible light-induced reactions of a suspension of titanium dioxide (TiO(2)) in water were investigated using electron paramagnetic resonance (EPR) coupled with the spin-trapping technique. Increased levels of both hydroxyl (˙OH) and superoxide anion (˙O(2)(-)) radicals were detected in TiO(2) rutile and anatase nanoparticles (50 nm). The intensity of signals assigned to the ˙OH and ˙O(2)(-) radicals was larger for the anatase phase than that originating from rutile. Moreover, illumination with visible (nonUV) light enhanced ˙O(2)(-) formation in the rutile phase. Singlet oxygen was not detected in water suspension of TiO(2) neither in rutile nor in anatase nanoparticles, but irradiation of the rutile phase with visible light revealed a signal, which could be attributed to singlet oxygen formation. The blue part of visible spectrum (400-500 nm) was found to be responsible for the light-induced ROS in TiO(2) nanoparticles. The characterization of the mechanism of visible light-induced oxy radicals formation by TiO(2) nanoparticles could contribute to its use as a sterilization agent.     

Characterization of paramagnetic species in N-doped TiO2 powders by EPR spectroscopy and DFT calculations

Electron paramagnetic resonance (EPR), X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT) calculations are combined for the first time in an effort to characterize the paramagnetic species present in N-doped anatase TiO2 powders obtained by sol-gel synthesis. The experimental hyperfine coupling constants are well reproduced by two structurally different nitrogen impurities: substitutional and interstitial N atoms in the TiO2 anatase matrix. DFT calculations show that the nitrogen impurities induce the formation of localized states in the band gap. Substitutional nitrogen states lie just above the valence band, while interstitial nitrogen states lie higher in the gap. Excitations from these localized states to the conduction band may account for the absorption edge shift toward lower energies (visible region) observed in the case of N-doped TiO2 with respect to pure TiO2 (UV region). Calculations also show that nitrogen doping leads to a substantial reduction of the energy cost to form oxygen vacancies in bulk TiO2. This suggests that nitrogen doping is likely to be accompanied by oxygen vacancy formation. Finally, we propose that the relative abundance of the two observed nitrogen-doping species depends on the preparation conditions, such as the oxygen concentration in the atmosphere and the annealing temperature during synthesis.     

Hydride ion as photoelectron donor in microporous crystal

Atomic hydrogen (H0) and trapped electrons generated by UV illumination (lambda approximately 330 nm) at 4 K were observed using electron paramagnetic resonance (EPR) in a 12CaO.7Al2O3 (C12A7) crystal heated in a hydrogen atmosphere. The concentration ratio of generated H0 to the electrons encaged in the subnanometer-sized cages of C12A7 (F+ centers) is almost 1:1, providing direct evidence that a hydride ion, H-, accommodated in the cage by the heat treatment was dissociated to a pair of an H0 and an electron by a UV photon: H- --> H0 + e- (F+). After annealing at 300 K, H0 was completely annihilated, while approximately 60% of the trapped electrons survived. The remaining electrons can hop between neighboring cages and give electrical conductivity to C12A7. The hyperfine splitting of the EPR spectrum of H0 in C12A7 (48.6 mT) is 4% smaller than that of the neutral hydrogen atom (50.6 mT), implying that H0 is trapped at the interstitial sites among the cages.     

Free-radical pathways in the decomposition of ketones over polycrystalline TiO(2): the role of organoperoxy radicals.

The oxidative decomposition of various ketones (including acetone, 2-butanone, 4-heptanone, cyclopentanone and cyclohexanone) over dehydrated TiO(2) (P25) powder is investigated by electron paramagnetic resonance (EPR) spectroscopy. For the first time, a series of thermally unstable radical intermediates are observed both on the activated and reduced TiO(2) surface, depending on the adopted experimental conditions. These radical intermediates are identified as organoperoxy-based species of general formula ROO(.-) and RCO(3) (.-). They are formed by reaction of photogenerated charge carriers (either trapped electrons or trapped holes) with the adsorbed ketones in the presence of molecular oxygen. The organoperoxy intermediates are thermally unstable and decompose at temperatures in the region of 180-250 K. This work demonstrates that free-radical pathways involving both organoperoxy and superoxide radicals can be responsible for the thermal- and photodecomposition of ketones over polycrystalline TiO(2) (P25).     

Photochemical charge transfer and trapping at the interface between an organic adlayer and an oxide semiconductor

Identification of charge transfer and trapping sites on semiconducting oxide surfaces is of fundamental importance in furthering the field of heterogeneous photocatalysts. Using scanning tunneling microscopy, electron energy loss spectroscopy, and photodesorption, we observed both electron trapping and hole transfer events on the (110) surface of TiO2 rutile. UV irradiation of a saturated monolayer of trimethyl acetate (TMA) on TiO2(110) at room temperature resulted in hole transfer to the carboxylate group, followed by (CH3)3C-COO bond cleavage and desorption of CO2 and isobutene/isobutane. Hole transfer to TMA proceeded in the absence of a gas-phase electron scavenger (which is typically O2) because the accompanying photogenerated electrons could be trapped at the surface as Ti3+ cations bound to bridging OH groups. The extent of electron trapping, gauged by electron spectroscopy, correlated directly with the yields of photodesorption fragments resulting from the hole transfer channel. Charge at the Ti3+ sites was titrated in the dark via a reaction between O2 and the Ti3+-OH groups.     

A bioinspired construct that mimics the proton coupled electron transfer between P680*+ and the Tyr(Z)-His190 pair of photosystem II

A bioinspired hybrid system, composed of colloidal TiO2 nanoparticles surface modified with a photochemically active mimic of the PSII chlorophyll-Tyr-His complex, undergoes photoinduced stepwise electron transfer coupled to proton motion at the phenolic site. Low temperature electron paramagnetic resonance studies reveal that injected electrons are localized on TiO2 nanoparticles following photoexcitation. At 80 K, 95% of the resulting holes are localized on the phenol moiety and 5% are localized on the porphyrin. At 4.2 K, 52% of the holes remain trapped on the porphyrin. The anisotropic coupling tensors of the phenoxyl radical are resolved in the photoinduced D-band EPR spectra and are in good agreement with previously reported g-tensors of tyrosine radicals in photosystem II. The observed temperature dependence of the charge shift is attributed to restricted nuclear motion at low temperature and is reminiscent of the observation of a trapped high-energy state in the natural system. Electrochemical studies show that the phenoxyl/phenol couple of the model system is chemically reversible and thermodynamically capable of water oxidation.     

Solid-solid interface formation in TiO2 nanoparticle networks

Aiming at a comparison of microstructure and paramagnetic properties of mesoporous TiO(2) nanoparticle networks, we subjected entirely different TiO(2-x) precursor structures to vacuum annealing. The transformation of an amorphous TiO(2-x) gel--obtained by sol-gel processing of an ethylene glycol-modified titanium precursor--into a network of interconnected anatase nanocrystals was explored by means of X-ray diffraction, nitrogen sorption, and electron microscopy. Crystalline junctions between the particles emerge from temperature treatment. This process of particle network formation is different from that related to the vapor phase grown anatase nanocrystals where particle-particle interface formation is induced by contact with water. It was found that, after annealing up to 873 K and controlled sample purification in oxygen atmosphere, both types of samples exhibit high concentrations of particle-particle interfaces and comparable properties in terms of surface area, porosity, and microstructure. With electron paramagnetic resonance (EPR) we observed on nonstoichiometric TiO(2-x) networks an identical type of subsurface defect which is related to the presence of solid-solid interfaces.     

Origin of photoactivity of nitrogen-doped titanium dioxide under visible light

Nitrogen-doped titanium dioxide (N-TiO2), a photocatalytic material active in visible light, has been investigated by a combined experimental and theoretical approach. The material contains single-atom nitrogen impurities that form either diamagnetic (Nb-) or paramagnetic (Nb*) bulk centers. Both types of Nb centers give rise to localized states in the band gap of the oxide. The relative abundance of these species depends on the oxidation state of the solid, as, upon reduction, electron transfer from Ti3+ ions to Nb* results in the formation of Ti4+ and Nb-. EPR spectra measured under irradiation show that Nb centers are responsible for visible light absorption with promotion of electrons from the band gap localized states to the conduction band or to surface-adsorbed electron scavengers. These results provide a characterization of the electronic states associated with N impurities in TiO2 and, for the first time, a picture of the processes occurring in the solid under irradiation with visible light.     

Inhibition of Electron-Hole Recombination in Substitutionally Doped Colloidal Semiconductor Crystallites

The subtitutional doping of 120-Å- sized TiO₂ Particles with Fe( III )ions has a profound effect on the charge carrier recombination time in this colloidal semiconductor. In undoped particles, the mean lifetime of an electronhole pair is ca. 30 ± 15 ns. Doping with 0.5% Fe( III ) drastically augments the charge-carrier lifetime which is extended to minutes or hours. The slow character of the recombination dynamics in Fe( III )-doped colloids was confirmed by laser photolysis using the characteristic optical of electrons in TiO₂ to monitor the time course of the reaction. EPR studies showed the Fe( III ) ions to enter the host lattice on Ti( IV ) sites, charge compensation taking place through the formation of oxygen vacancies. Valence-band holes produced under band-gap excitation react with these centers it the bulk forming Fe( IV ), the conduction band electrons being trapped by Ti( IV ) at the particle surface. Presumably, the spatial separation of the trapped electron and hole sites inhibits their recombination.     

Tunable photoluminescent and cathodoluminescent properties of ZnO and ZnO:Zn phosphors

ZnO and ZnO:Zn powder phosphors were prepared by the polyol-method followed by annealing in air and reducing gas, respectively. The samples were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), X-ray photoelectron spectra (XPS), electron paramagnetic resonance (EPR), and photoluminescence (PL) and cathodoluminescence (CL) spectra, respectively. The results indicate that all samples are in agreement with the hexagonal structure of the ZnO phase and the particle sizes are in the range of 1-2 microm. The PL and CL spectra of ZnO powders annealed at 950 degrees C in air consist of a weak ultraviolet emission band (approximately 390 nm) and a broad emission band centered at about 527 nm, exhibiting yellow emission color to the naked eyes. When the sample was reduced at the temperatures from 500 to 1050 degrees C, the yellow emission decreased gradually and disappeared completely at 800 degrees C, whereas the ultraviolet emission band became the strongest. Above this temperature, the green emission ( approximately 500 nm) appeared and increased with increasing of reducing temperatures. According to the EPR results and spectral analysis, the yellow and green emissions may arise from the transitions of photogenerated electron close to the conduction band to the deeply trapped hole in the single negatively charged interstitial oxygen ion (Oi(-)) and the single ionized oxygen vacancy (V.O) centers, respectively.     

One-electron redox processes during polyoxometalate-mediated photocatalytic reactions of TiO(2) studied by two-color two-laser flash photolysis

The one-electron redox processes of several compounds during polyoxometalate (POM)-mediated photocatalytic reactions of TiO(2) were investigated using the two-color two-laser flash-photolysis technique. The efficiency of the one-electron oxidation of aromatic sulfides by the trapped hole (h(tr) (+)) or the surface-bound OH radical (OH(s) (.)) is found to be significantly enhanced due to electron transfer from the conduction band (CB) of TiO(2) to the POM. The efficiency of the electron transfer from the CB of TiO(2) to the POM decreases in the order H(2)W(12)O(40) (6-) < SiW(12)O(40) (4-) < PW(12)O(40) (3-), that is, it depends on the reduction potential (E(red)) of the POMs. Electron injection from PW(12)O(40) (4-) in the excited state (PW(12)O(40) (4-*)) to the CB of TiO(2) was clearly observed using the two-color two-laser flash-photolysis technique. Storage of electrons in the TiO(2)/PW(12)O(40) (3-)/methyl viologen (MV(2+)) ternary system was also achieved upon two-color two-laser irradiation.     

Energy transfer on the MgO surface,monitored by UV-induced H2 chemisorption

Surface anions on edges (4-coordinated = 4C) and on corners (3-coordinated = 3C) of cubic MgO nanoparticles exhibit UV resonance absorptions around 5.5 and 4.6 eV, respectively. After monochromatic excitation of either site the electron paramagnetic resonance (EPR) spectrum exhibits exclusively signal components related to 3-coordinated O- radicals (O-(3C), electron hole centers), which are perfectly bleached by H(2) addition. The disappearance of the O-(3C) EPR signal components is paralleled by a depletion of the UV resonance absorption of the 3-coordinated O(2-) only and the appearance of one single band in the OH stretching region of the IR spectrum. Obviously the sites of UV excitation and subsequent UV induced surface reaction with H(2) are not the same. This may coherently be explained in terms of mobility of the exciton (O(2-)(4C)* or--after ionization--of the corresponding electron hole O-(4C) along the edge where it was created. Finally the mobile state is trapped at a corner site where the O(3C)H group is formed.