Shedding Light on Aging of N-Doped Titania ... - ACS Publications

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The Journal of Physical Chemistry

Shedding Light on Ageing of N – Doped Titania Photocatalyst Alexey Tarasova,с, Anton Minnekhanovb, German Trusovc, Elizaveta Konstantinovab, Alexandr Zyubina, Tatiana Zyubinaa, Alexey Sadovnikove, Yury Dobrovolskya, Eugene Goodilinc,d,e,* a

Laboratory of Solid State Ionics, Institute of Problems of Chemical Physics RAS,

Academician Semenov avenue 1, Chernogolovka, 142432, Russia. E-mail: [email protected] b

Department of Physics, Lomonosov Moscow State University; Lenin Hills, Moscow,

119991, Russia c

Department of Chemistry, Lomonosov Moscow State University; Lenin Hills,

Moscow, 119991, Russia d

Faculty of Materials Science, Lomonosov Moscow State University; Lenin Hills,

Moscow, 119991, Russia e

Kurnakov Institute of General and Inorganic Chemistry, Leninsky prospect 31,

Moscow, 119991, Russia Author Information Corresponding author * Faculty of Materials Science, Lomonosov Moscow State University; Lenin Hills, Moscow, 119991, Russia, Tel.: +74959394729, E-mail: [email protected]

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Abstract A detailed analysis of nitrogen dopant behavior in nanostructured microspheres of the TiO2 photocatalyst obtained by the Thermally Assisted Reactions in Aqueous Sprays method has been performed for the first time using EPR, XPS and UV-vis and supported by theoretical simulation of possible defect structures. The nitrogen species were found to undergo the N • to N- transformation during samples storage under different conditions with its activation energy of about 0.45 eV. Three main possible evolution pathways for the dopant state were identified

and

discussed.

It

was

established

that

the

most

probable

transformation consists of migration of an oxygen vacancy site to an interstitial nitrogen atom followed by the formation of a non-paramagnetic substitution nitrogen center. Possible diffusion routes of oxygen vacancy and corresponding energy barriers were estimated and found to be in agreement with experimental observations.

Keywords Nitrogen doping, N-TiO2, catalyst degradation, oxygen vacancy migration


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Introduction Nowadays, titanium dioxide attracts deep interest as a promising nanomaterial for photovoltaic, water photospliting and air purification1-4. Unfortunately, widely expected practical applications of TiO2-based photocatalysts are restricted by a heavy requirement of operation using artificial UV light sources. Sunlight is a natural inexpensive source of energy for photocatalysis, but only about 3% of solar spectrum can be absorbed by pristine TiO2 due to its large band gap (e.g. 3.2 eV for anatase). Since Asahi5 has shown the N-doping of titania to be a facile approach to overcome this drawback, many strategies have been developed to obtain doped titania, including ion implantation, magnetron sputtering, plasma and laser treatment, mechanoactivation6-10 hydrothermal and solvothermal treatment, sol-gel routes, TiN oxidation, high temperature sintering under ammonia atmosphere11-15. A number of studies were devoted to investigate the features of doping procedure like the nitrogen influence on thermodynamic stability of titania phase modifications16, determination the form of nitrogen species in the titania lattice17, dopant’s influence on optical18 and functional properties of doped titania19. In the most, theoretical research is focused on the changes of semiconductor’s electron structure caused by doping20, the dopant role in photoinduced charge carriers recombination processes21, impurities influence on lattice defects stability22 but only a few of them were dedicated to explore a storage stability and operation durability of such materials at different conditions, that is of vital importance for future practical applications. Spadavecchia23 has studied the ageing (evolution of optical, paramagnetic and photocatalytical properties over time) of N-doped titania samples, prepared by a hydrothermal treatment using different nitrogen precursors (NH3, triethylamine and urea). Faster ageing observed for the samples prepared using NH3 was explained by prevailing surface localization of the defects and their consequent lower stability upon



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the contact with O2/humidity, while organic precursors were assumed as less volatile and therefore forming more “permanent” defective sites during the synthesis. The high - temperature calcination of nitrogen doped titania was investigated using radioactive nuclear probes by Mihara24. He has reported relatively higher stability of substituted N - impurities in comparison with interstitial nitrogen atoms. D’Arienzo25 has concluded, that a high temperature treatment causes transformation of the interstitial N into a substitution form with no impact on its photocatalytical properties. In this paper we report for the first time the results of complex analysis of nitrogen dopant behavior in TiO2 photocatalyst during its artificial ageing for different time under ambient temperatures. The study of final states of the material performed using EPR, XPS and UV-vis techniques is certainly in a good agreement with theoretical DFT simulation of possible defect structure variants. Finally, the obtained results shed light on post-synthetic stability of N-TiO2 materials and the processes proceeding in the route of its storage.

Experimental Nitrogen-doped TiO2 powders with an average microsphere diameter of about 0.5-3 µm were synthesized by the method of thermally assisted reactions in aqueous sprays (TARAS) using urea as a nitrogen dopant precursor26 (see ESI). The studied samples consist of anatase mixed with rutile in the ratio of 60% to 40%. In this work, we examined the influence of storage of the sample obtained almost two years ago at 1% of urea concentration in the initial hydrolyzing solution annealed at 1000oC (denoted TON1-S) and two as - prepared samples, obtained at different concentration of urea in the initial hydrolyzing solution: 0.5% (denoted TON05) and 1% (denoted TON1) with the same calcination temperature. To explore the temperature impact on storage stability, a


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portion of TON1 sample was stored in the dark at 80oC for totally 5 weeks (denoted as TiON1-T). All studied samples were stored and treated in powder form. Electron paramagnetic resonance (EPR) spectra were recorded at room temperature (unless indicated other) by the standard Bruker EPR spectrometer ELEXSYS-500 (Xband, sensitivity of about 5•1010 spin/G, modulation frequency 100 kHz). Computer fitting of the spectra were obtained using the Simfonia program. To evaluate the gvalues of EPR signals and spin concentration in the samples, a standard of MgO matrix with Mn2+ ions and a CuCl2•H2O standard with known spin concentration were used, respectively. The studied samples were placed in different capillar tubes, maintained in cylindrical cavity for the measurements in darkness or in optical cavity for the measurements under illumination with a 100 W halogen lamp in the spectral range of ∆λ = 270-900 nm (1.38 – 4.6 eV). UV-vis absorption spectra were obtained by means of the UV/Vis spectrometer Lambda 35 (Perkin-Elmer, USA) operated in a diffusion reflectance mode in the range of 1901100 nm. High resolution X-ray photoelectron spectra (XPS) were recorded using the Kratos AXIS Ultra DLD spectrometer with an incorporated Al Ka (hν = 1486.6 eV) 120 W X-ray source on sample area of 300x700 µm2. The binding energy was calibrated by referencing the C 1s peak to 285eV. Because of low conductivity of the samples, a lowenergy electrons moving in a spiral path in a magnetic field were used as positive charge neutralizer. Before XPS measurements, the samples were kept in vacuum in order to clean the surface from the physically adsorbed species. The DFT calculations of N atoms migration in the anatase crystal lattice of TiO2 were performed with periodical boundary conditions and plane-wave expansions using the generalized gradient approximation in the Perdew–Burke–Emzerhof form (GGA-PBE). The Vienna ab initio simulation package was the base of all the calculations (VASP)27-32.



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Within the VASP modeling, a projector augmented plane wave (PAW) basis set was used with the corresponding pseudopotential and the PBE functional. The plane-wave cutoff energy was set as 400 eV. For simulation of nitrogen and vacancy migration in bulk anatase, (TiO2)32 and (TiO2)72 supercells were used. For the evaluation of the transition states, the nudged elastic band method implemented in VASP was applied.

Results and Discussion Long – term ageing under ambient conditions The typical EPR spectrum of N-doped titania sample is shown in fig 1a. EPR lines detected in the samples can be ascribed to the O2- and N• paramagnetic centers33. In order to separate a contribution from O2• radicals, N• concentration was monitored as peak-to-peak intensity of right-sided satellite peak as shown in the inset. The EPR spectra of TON1-S storage sample, recorded after 8, 12 and 21 months of its synthesis, demonstrate strong depletion of the N• signal in comparison with the initial species. The value of corresponding N• signals are shown in fig. 1b, an exponential fit gives an attenuation coefficient R0(20 oC) = -0.008 day-1. An eightfold decrease in N• signal intensity, for the first glance, can be attributed to the exhausting of N species when leaking out from the sample. This would mean a dramatic impairment of doped titania functional properties under visible illuminations since N species in the TiO2 lattice are believed to be responsible for visible light harvesting22. UV-vis diffusion reflectance spectra of TON1-S (fig.1c), recorded initially after preparation and after 21 months of its synthesis, demonstrate an absorbance shoulder specific for N-doped titania in the visible region of 420-550 nm contrary to pure titania. The absorption in this visible region is usually attributed to electron excitation in a conductive band from an extrinsic energy state of nitrogen in the titania lattice (N2p → Tid)21 and its relative intensity correlates with concentration of lattice defects.


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Surprisingly, the intensity of visible absorption, measured after the storage, does not undergo significant changes as compared to the EPR data. In view of the fact that different nitrogen impurities are presumed to contribute to visible light absorption, the observed decrease of N• concentration is likely to be caused by some internal processes, associated with a transformation of paramagnetic N• species into nonparamagnetic forms. For a more careful examination of the observed effect, we have repeated storage experiments for a shorter period of time to corroborate the UV-vis DSR and EPR data with the XPS technique.

Figure 1.

Short – term storage samples The N• peak intensities of TON05 and TON1 samples do not show significant impairment during the 35 days storage at ambient temperature (fig.2a). The high ACS Paragon Plus Environment


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intensity of first peaks for both the samples (2387 a.u. and 1974 a.u.) as well as deviations around the average value (375 a.u. and 950 a.u.) observed during the storage, were explained by illumination of the sample by the visual light during the preparation and mounting of the probes. The results of EPR measurements, done after intentional illumination of EPR probes by halogen lamp before EPR measurements (N• peak intensities are 2421 a.u. and 2489 a.u., shown at fig.2a as red points) fully support this statement.

Figure 2. The visible absorption of TON05 and TON1 samples, as expected, demonstrate no changes, similarly to the TON1-S stored sample (fig. 2b). Thus, the obtained results demonstrate a strong correlation between loading of the initial nitrogen precursor 0.5 wt% and 1 wt%), the N• peak intensity (375 a.u. and 950 a.u.) and visible absorption (almost twofold for TON1 than for TON05), but no signal impairment during short term ageing at ambient temperature was found. A different situation is observed in the case of short term ageing at slightly increased temperature (80 oC). In spite of the almost permanent shoulder of visible absorption, the N• peak intensity of TON1-T sample, shown at fig.3a as solid black squares, have a clear trend to decrease during 35 days of experiment (fig. 3), even below the initial value of TON05 sample. The first elevated point at 1850 a.u. is assumed to have the artefact nature caused by sample preparation (the intentional illumination of TON1-T


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sample leads to intensity increase up to 2235 a.u., as shown with red circles). For the fitting purpose, the first point was substituted by average intensity of N• peak with respect to different mass of the sample (1045 a.u., shown as red star). Hence, the N• peak intensity with a corrected first point demonstrates almost fivefold impairment of the signal throughout 35 days (the attenuation coefficient R0(80oC) = -0.1 day-1 for exponential fitting).

Figure 3. The temperature dependence of attenuation coefficients, observed for the TON1-S and TON1-T samples (-0.008 day-1 at 20oC and -0.1 day-1 at 80 oC) allows to assume that the possible processes for the observed EPR signal impairment have a remarkable activation energy. Assuming the Arrhenius nature of this process, the activation energy Ea of the N• transformation process proposed above was estimated as Ea=0.45 eV. The XPS technique is widely used to detect the amount and the form of nitrogen in the titania lattice34. However an assignment of XPS peaks to certain N-forms is still under debate, N 1s core level peaks at 396–397 eV are usually ascribed to substitutional nitrogen, while the peaks at ~400 eV to nitrogen in interstitial site22. The XPS spectra of TON1 and TON1-T samples, shown at fig.4, demonstrate a single peak centered at binding energy close to the one ascribed for a nitrogen interstitial site. The peak position slightly shifts after the thermal treatment to lower energy values while



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the total nitrogen concentration, calculated from XPS data, remains unchanged with respect to the overall sample weight.

Figure 4. Thus EPR, UV-vis DRS and XPS data support the suggestion that the depletion of the N• signal during has to be associated with transformation into another nitrogen forms rather than exhaustion out of the titania lattice.

Mechanism of depletion of N• signal To explain the N• annihilation phenomena, we suggest three possible routes as shown below: N2 formation via N•i radicals recombination (1), N2 formation via recombination of N•i with N-s(2) and N•i reduction into N-i (3). Ni• + Ni• N2(i)

(1)

Ni• + N-s N2(s)

(2)

Ni• + VO N-s

(3)

All the routes should be limited by at least two factors: the diffusion of corresponding species in the titania lattice and the abundance of the species in the lattice. In order to determine the most plausible route of N• degradation, a relative abundance for N•(i), N-(s), and VO species and corresponding migration energies were estimated using the EPR technique supported by DFT GGA simulation.


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At first, we have compared the processes (1) and (2). While substitutional nitrogen atoms N-s are immobilized in the titania lattice and only interstitial N•i diffusion can drive the processes, the probability of both the processes is to be defined by N•i and N-s relative concentrations. In accordance with the XPS measurements (fig. 4a), the concentration of substitutional nitrogen N-s in as - prepared samples is negligibly low. Therefore the formation of interstitial N2, similar to the samples described elsewhere35, seems to be more feasible in the first suggested route. The competition of (1) and (3) processes depends on two parameters: relative concentration and diffusion abilities of N•i and VO species in the lattice. Since the formation of oxygen vacancies is coupled with the formation of paramagnetic Ti3+ center, the relative concentration of N•i and VO species in the sample can be estimated from the intensity of their EPR signals. The EPR spectrum of TON1 sample, measured at 115 K (to compensate short relaxation time of Ti3+ centers) is shown at fig. 5. As can be seen, the new large EPR signal is detected with g1=1.996, g2=1.925. According to the literature data we can ascribe this EPR signal to the Ti3+ / oxygen vacancy paramagnetic centers33. The concentrations of N• and Ti3+ centers, calculated by double integration of the signal, are in the ratio of 1 to 125. Thereby, the third route of N•i to N-s transformation, caused by the interaction with oxygen vacancy is most likely to be responsible for the observed EPR signal depletion.

Figure 5.



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The N• to N- transformation is described in many studies as an inherent part of doped titania formation process36 or as a result of high temperature treatment25. However, to the best of our knowledge, we report for the first time a continuation of this process in the prepared N-doped titania during its long-term storage.

DFT simulations of N• to N- transformation The interaction of N• and VO should include its approaching by migration of at least one component, electron transfer and proper lattice rearrangement. In order to elucidate the features of these processes, they were simulated using the DFT-GGA approach. At first, we estimated the possibility of electron transfer from a single oxygen vacancy to a nitrogen atom. Without electron transfer, the NO fragment has one unpaired electron and a vacancy has two electrons, so the system in the whole has three unpaired electrons (m=3). In this case electrostatic potentials on N and O atoms have approximately the same values as for the system with no vacancy (deviations less than 0.1 eV). After electron transition from the vacancy to NO, the number of unpaired electrons changes from 3 to 1, electrostatic potential values on N and O atoms are shifted on 1.1 and 0.9 eV, accordingly, and the total energy of the system is lowered on 1-2 eV, depending on the VO – N(i) distance (Fig. 6). However, the total energy of the system with substitutional nitrogen (Fig. 6, TiO2_Ns) is ~2 eV lower that any state with an electron transfer. These results give us two important conclusions: (i) the electron transfer should be taken into account during calculations of species migration and (ii) an approaching of N• and VO leads to nitrogen trapping by the oxygen vacancy with stable N(s) formation.


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Figure 6.

The diffusion of nitrogen in a perfect anatase lattice was calculated by Xiao37 using the DFT-GGA approach. The migration barriers for three identified unequal paths were identified as EA = 1.43 eV, EB = 1.94 eV and EC = 1.83 eV. In our calculations, the influence of established above electron transfer from oxygen vacancy to interstitial nitrogen was found to have a significant impact on nitrogen migration barriers. The three diffusion paths and corresponding energy barriers for a N- species migration are shown in Fig. 7



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Figure 7. The migration barriers for d1-d2 and d2-d3 are close to those reported by Xiao, while the d2-d4 is significantly higher (2.1 eV) than one calculated without vacancy influence (1.43 eV). Recalculated without vacancy d2-d4 migration barrier was found 1.42 eV. The reason of this difference becomes clear when considering the actual forms of nitrogen species during migration with and without oxygen vacancy. In the absence of vacancy, nitrogen moves through the paths d1-d2 and d2-d3 with destruction of N-O bonds in the barrier region (EB = 1.94 eV and EC = 1.83 eV), while it forms the O-N-O angled fragment and that decreases its relative energy (EA = 1.43 eV) through the d2-d4 path. In the presence of an oxygen vacancy and if an electron transfer from the oxygen vacancy to this nitrogen atom occurs, the transferred electron gives an antibonding effect for the O-N bonds, therefore nitrogen moves through the d2-d4 path destructing the N-O bonds and increasing the migration barrier (2.1 eV). It should be mentioned, that the lower migration barrier in the anatase (d2-d4 path) corresponds to the movement along the diagonal of the TiO2Ti rhomb with the shortest O-O distance in the TiO6 octahedron (2.5 Ǻ in comparison with 2.8Ǻ and 3.0Ǻ). These ACS Paragon Plus Environment

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rhombs form flat noncrossing ribbons in the lattice and migration through the d2-d3 path causes only quasi one-dimensional movement. Any transfer from one ribbon to another one is associated with overcoming other two barriers of about 2 eV. In the rutile form, the corresponding O-O distances are close to 2.6 Ǻ, 2.8 Ǻ and 3.0 Ǻ and thus migration barriers are likely to be even higher than in the anatase. Moreover, the TiO2Ti rhombs in rutile with the shortest O-O distance are not connected directly and do not form one dimensional transfer ribbons with minor migration energy like in anatase. Thus, the nitrogen migration in anatase or rutile lattices through oxygen defects is associated with overcoming the barriers of about 2 eV and is much higher compared to the activation energy found for aged samples that is as low as 0.45 eV in the experiments. Alternatively, the oxygen vacancy migration can also cause approaching of N• and oxygen vacancy with subsequent N- species formation caused by electron transfer or nitrogen trapping. The vacancy migration barriers in rutile were calculated to be from 0.6 eV to 1.7 eV for different directions38-40. In the present study, we calculated the migration barriers in anatase using a (TiO2)72 cluster. TiO6 octahedron in anatase is distorted giving C2v symmetry thus it has four different vacancy positions (1, 2, 3, 4) with nearly similar relative energy (±0.1 eV) as shown in fig. 8a.

Figure 8. ACS Paragon Plus Environment


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The migration barrier between the positions of 1 and 2 (0.18 eV) is significantly lower than for 1-3 and 1-4 (1.14 eV and 1.9 eV) paths (fig. 9). It can be seen, that the oxygen atoms in the 1 and 2 positions form the TiO2Ti rhombs, connected in flat noncrossing ribbons (shown at fig. 8b) spread in two mutually perpendicular directions (010) and (100), shown in fig 8b. In the anatase lattice, these ribbons can act as channels of high oxygen vacancy conductivity with a low activation energy (0.18 eV). To realize three dimensional migration in such a lattice, the oxygen vacancy needs to overcome higher energy barriers (1.14 eV or 1.9 eV) and switch between the channels. However, already the migration along ribbons is sufficient to cause the observed phenomena of doped titania ageing. It should be mentioned, that, in rutile, the oxygen positions, correspond to a low energy transition, do not form continuous ribbons because of different TiO6 octahedron orientation. The vacancy can only jump/swing between two neighbor positions but do not migrate through the rutile lattice. Thus, the above proposed mechanism of N• transformation can occur only in the anatase form of the samples.

Figure 9.


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The migration barrier of oxygen diffusion, calculated by DFT-GGA approach, doesn’t differ drastically from the prediction of activation energy given from experiments of EPR signal depletion. Hence, the depletion of N• EPR signal, observed during the ageing of N-doped titania at elevated temperature, is likely caused by its interaction with oxygen vacancy followed by paramagnetic substitutional N- formation. This conclusion corroborates the results of XPS measurements, where the position of nitrogen shifts after calcination towards the lower binding energy as assumed to be typical of substitutional species. Our findings are in good agreement with calculations done by Tsetseris41 using the same DFT-GGA approach with a smaller TiO2 supercell. The different rate of ageing, observed by Spadavecchia23 on N-doped titania samples, obtained from different nitrogen precursors, can be explained by different oxygen vacancy concentration in lattice, defined by various reducing ability of nitrogen source. It is worth to notice, that during samples ageing we did not observe any significant changes in photocatalytic activity of the doped titania samples under both either uvvisible and or visible irradiations (see Supporting information). This fact agrees well with the assumption that all forms of nitrogen in titania lattice contribute to its photocatalytical performance and particular transformation of N• into N- cause no impact of doped materials functional properties. Thus, it is arguable that the ageing of nitrogen doped titania at moderate temperature (up to 80oC) doesn’t impact its functional properties, in spite of continuation of internal processes of defects interactions.

Conclusions The results of our complex post-synthesis study of N-doped TiO2 show, that the storage of doped titania samples at moderate temperature is accompanied by interaction of nitrogen impurities with lattice defects. Full nitrogen concentration in the TiO2 lattice remains unchanged although the concentration of interstitial paramagnetic N• centers in



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the lattice decreases with time. The transformation of N• species into non-paramagnetic N- state was attributed to N• capture by oxygen vacancy, caused by oxygen vacancy diffusion in the anatase lattice. The (010) and (100) directions were determined as the most possible diffusion directions with migration barriers of ~0.2 eV. The photocatalytic activity of the samples under visible irradiation doesn’t undergo any significant changes during the ageing. Thus, it is arguable that the storage of nitrogen doped titania at moderate temperature (up to 80oC) doesn’t impact its functional properties, in spite of continuation of internal processes of defects interactions. However any storage at room temperature would guarantee preserving high enough activity of nanostructured titania.


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Acknowledgments This work was supported by RSF grant № 14-23-00218. The EPR measurements were performed using the facilities of the Collective Use Center at the Moscow State University and in frame of the Program of development of M.V. Lomonosov Moscow State University.

Supporting materials Material’s

morphology

(SEM)

and

photocatalytic

activity

(crystal

violet

photodegradation). This material is available free of charge via the Internet athttp://pubs.acs.org.



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Reference (1) O'Regan, B.; Grätzel, M.; Low-cost, high-efficiency solar cell based on dyesensitized colloidal TiO2 films; Nature 1991, 353, 737-740 (2) Selli, E.; Chiarello, G.-L.; Quartarone, E.; Mustarelli, P.; Rossetti, I.; Forni, L.; A photocatalytic water splitting device for separate hydrogen and oxygen evolution; Chem. Commun. 2007, 5022–5024 (3) Fujishima,

A.; Hashimoto,

K.; Watanabe,

T.;

TiO2

Photocatalysis:

Fundamentals and Applications; Bkc, Tokyo, 1999 (4) Ao, C.H.; Lee, S.C.; Indoor air purification by photocatalyst TiO2 immobilized on an activated carbon filter installed in an air cleaner; Chem. Eng. Sci. 2005, 60 (1), 103–109 (5) Asahi, R.; Morikawa, T.; Ohwaki, T.; Aoki, K.; Taga, Y.; Visible-light photocatalysis in nitrogen-doped titanium oxides; Science 2001, 293(5528), 269– 271 (6) Ghicov, A.; Macak, J. M.; Tsuchiya, H.; Kunze, J.; Haeublein, V.; Frey, L.; Schmuki, P.; Ion implantation and annealing for an efficient N-doping of TiO2 nanotubes; Nano Lett. 2006, 6, 1080-1082 (7) Mwabora, J. M.; Lindgren, T.; Avendano, E.; Jaramillo, T.F.; Lu, J.; Lindquist, S.E.;

Granqvist,

C.G.;

Structure,

composition

and

morphology

of

photoelectrochemically active TiO2-xNx thin films deposited by reactive magnetron dc sputtering; J. Phys. Chem. B 2004, 108, 20193–20198 (8) Hong, Y.; Bang, C.; Shin, D.; Uhm, H.; Band gap narrowing of TiO2 by nitrogen doping in atmospheric microwave plasma; Chem. Phys. Lett. 2005, 413, 454


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Figures captures Figure 1. a) Typical EPR spectrum of TON samples in darkness at room temperature; Long-term measurements of storage sample TON1-S: b) the N• peak intensity from EPR spectra recorded in the darkness; c) UV-vis diffusion reflectance spectra in comparison with spectrum of pristine TiO2. Figure 2. Short-term measurements of TON05 and TON1 samples: a) the N• peak intensity extracted from EPR spectra recorded in the darkness; b) UV-vis diffusion reflectance spectra in comparison with spectrum of pristine TiO2. Figure 3. Short-term measurements of sample TON1-T, stored at 80oC: a) the N• peak intensity from EPR spectra recorded in the darkness; b) UV-vis diffusion reflectance spectra in comparison with spectra of TON05 sample and pristine TiO2. Figure 4. N 1s core level XPS spectra of TON1 and TON1-T samples. Figure 5. EPR measurements of TON1 sample at 115 K. Figure 6. The changes of system energy caused by electron transfer from oxygen vacancy to interstitial nitrogen atom at different VO – Ni positional relationship, compared to the system with substitution nitrogen. The VO – Ni distances are a – 7.35 Å, b – 3.07 Å, c – 2.54 Å, d – 2.45 Å. Figure 7. Different positions of nitrogen atom in the titania lattice and corresponding transition states energies. The Ti atoms are the blue circles, the O atoms are the red. Figure 8. (a) enlarged fragment of supercell, the numbers in the picture represent different oxygen sites (marked pink). (b) The oxygen diffusion channels (marked with pink circles and arrows) in anatase lattice. The Ti atoms are the blue circles, the O atoms are the red ones. Figure 9. The migration barriers between different possible vacancy positions in the anatase lattice. The Ti atoms are the blue circles, the O atoms are the red.



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TOC Graphic


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Figure 1. a) Typical EPR spectrum of TON samples in darkness at room temperature; 201x141mm (300 x 300 DPI)



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Figure 1. b) Long-term measurements of storage sample TON1-S: the N• peak intensity from EPR spectra recorded in the darkness; 203x142mm (300 x 300 DPI)


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Figure 1. c) UV-vis diffusion reflectance spectra in comparison with spectrum of pristine TiO2. 209x148mm (300 x 300 DPI)



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Figure 2.a) Short-term measurements of TON05 and TON1 samples: the N• peak intensity extracted from EPR spectra recorded in the darkness; 287x201mm (300 x 300 DPI)


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Figure 2.b) Short-term measurements of TON05 and TON1 samples: UV-vis diffusion reflectance spectra in comparison with spectrum of pristine TiO2. 296x209mm (300 x 300 DPI)



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Figure 3. Short-term measurements of sample TON1-T, stored at 80oC: a) the N• peak intensity from EPR spectra recorded in the darkness; 201x141mm (300 x 300 DPI)


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Figure 3.b) Short-term measurements of sample TON1-T, stored at 80oC: UV-vis diffusion reflectance spectra in comparison with spectra of TON05 sample and pristine TiO2. 296x209mm (300 x 300 DPI)



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Figure 4. N 1s core level XPS spectra of TON1 and TON1-T samples. 209x148mm (300 x 300 DPI)


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Figure 5. EPR measurements of TON1 sample at 115 K. 209x148mm (300 x 300 DPI)



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Figure 6. The changes of system energy caused by electron transfer from oxygen vacancy to interstitial nitrogen atom at different VO – Ni positional relationship, compared to the system with substitution nitrogen. The VO – Ni distances are a – 7.35 Å, b – 3.07 Å, c – 2.54 Å, d – 2.45 Å. 146x149mm (150 x 150 DPI)


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Figure 7. Different positions of nitrogen atom in the titania lattice and corresponding transition states energies. The Ti atoms are the blue circles, the O atoms are the red. 167x207mm (150 x 150 DPI)



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Figure 8. (a) enlarged fragment of supercell, the numbers in the picture represent different oxygen sites (marked pink). The Ti atoms are the blue circles, the O atoms are the red ones. 122x140mm (150 x 150 DPI)


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Figure 8. (b) The oxygen diffusion channels (marked with pink circles and arrows) in anatase lattice. The Ti atoms are the blue circles, the O atoms are the red ones. 171x178mm (150 x 150 DPI)



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Figure 9. The migration barriers between different possible vacancy positions in the anatase lattice. The Ti atoms are the blue circles, the O atoms are the red. 166x183mm (150 x 150 DPI)


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Shedding Light on Aging of N-Doped Titania ... - ACS Publications

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