Spectral Features of Photostimulated Oxygen Isotope Exchange and

Article pubs.acs.org/JPCC

Spectral Features of Photostimulated Oxygen Isotope Exchange and NO Adsorption on “Self-Sensitized” TiO2−x/TiO2 in UV−Vis Region Victor V. Titov, Ruslan V. Mikhaylov, and Andrey A. Lisachenko* Department of Physics, Saint-Petersburg State University, ul. Ul’yanovskaya, 1, Saint-Petersburg 198504, Russia S Supporting Information *

ABSTRACT: We have investigated the photoactivated O2 adsorption/desorption, the isotope equilibration (POIEq), and the isotope exchange (POIEx) in the O2−(TiO2−x/TiO2) (Degussa P-25) system, as well as the adsorption and POIEx in the NO−(TiO2−x/TiO2) systems. The photoactivation spectra of the hole centers Os− generation, of POIEq, and of the NO photoadsorption were measured in the 300−500 nm range using a continuously adjustable wavelength light source. The spectral characteristics of the NO photoadsorption are close to those of POIEq Both electron-donor and hole centers are manifested in these processes. For O2 photodesorption, CO oxidation, POIEx, and POIEq the hole centers Os− are responsible. In the case of O2 and NO photoadsorption both types of centers are active. The maxima of POIEq and of NO photoadsorption are placed near 420 nm beyond the edge of the TiO2 bandgap absorption. It was found that not only does the N18O−Ti16O2 exchange occurs under the illumination but also it can be activated by a preliminary sample illumination in vacuum as it was observed for POIEq. The photoactivation spectrum does not coincide with the absorption spectra of the F-type centers (F+ and F-centers, i.e., oxygen anion vacancies filled with one or two e−, respectively) and of Ti3+. The formation of the 2D structures, “core−shell” TiO2−x/TiO2, on the parts of the surface region and the decreasing of bandgap on these parts are supposed based on the kinetic data (the “fast” excitations in the bulk under UV irradiation and the “slow” excitations on the surface under the visible irradiation) and on the spectral data (maximum action is near 420 nm for both electronic and hole centers).

1. INTRODUCTION

they are considered to be responsible for the photocatalytic activity.6 Basing on scanning tunneling microscopy (STM) observations combined with density functional theory (DFT) results, He et al.10 concluded that intrinsic defects on anatase (101) are predominantly located in the subsurface layer and thus they appear to be less reactive than surface oxygen vacancies. When located just beneath the top surface layer, they could survive and steer photoexcited charge carriers to near-surface regions. The authors believe that potentially the propensity of anatase to form subsurface defects contributes to its superior photocatalytic properties. Komaguchi et al.11 used O2− in an EPR study as a molecular probe to explore the active sites including their environment on the TiO2 surface. They observed the electron transfer under vis illumination that leads to the concomitant formation of O2− and Ti3+ radicals. According to their opinion the active centers are a set of several sorts of F-type color centers having a relatively broad absorption spectrum centered around 460 nm. In a series of papers (see ref 12 and references therein) it was supposed that intrinsic F-centers are responsible for the photocatalytic

Titanium dioxide (TiO2) is widely used for photoelectric and photocatalytic conversion of the solar energy in the UV spectral region. In order to sensitize TiO2 in the visible region, TiO2 is usually doped with transition metals, nitrogen, sulfur, and other elements.1−4 However, many years ago Formenti et al.5 have shown by means of electron paramagnetic resonance (EPR) that O2− is formed on the irradiated undoped TiO2 surface in a broad range of wavelengths up to 520 nm, though no correlation with photocatalytic activity of TiO2 in the visible (vis) region was established. It was found using optical diffuse-reflectance spectroscopy (DRS) and UV (8.43 eV) photoelectron spectroscopy (UPS) that the reduction of TiO2 induces light absorption in the region 380 < λ < 2500 nm, (0.5 < hν < 3.2 eV); see ref 6. The absorption was ascribed partly to free electrons (a continuum at hν < 1.50 eV), to local centersTi3+ ionsand to F- and F+-type color centers.6 In TiO2-based photocatalysts the surface defects that act as trap sites for photoexcited charge carriers are quenched by an adsorption of gas molecules.7 The adsorbing oxygen heals the vacancies forming the O2− species near the filled vacancies.8,9 The exposure of TiO2−x to O2 or NO destroys partly such centers and results in the formation of specific O2, or N2O, NO, and NO2 adsorbed species.6 However, some colored centers survive, and © 2014 American Chemical Society

Received: May 22, 2014 Revised: August 29, 2014 Published: August 29, 2014 21986

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desorption and isotope exchange processes in systems O2− TiO2−x/TiO2 and NO−TiO2−x/TiO2. Special attention was paid to the spectral features of TiO2−x/TiO2 photoactivation. Thanks to a high-power light source equipped with a unique 1:1.8 aperture monochromator, a continuous wavelength scanning (instead of discrete spectral lines) was possible. This way the spectral kinetic parameters of the processes are determined. The study of the photoactivation spectra of molecular processes revealed an important role of a core−shell structures on the photocatalyst surface.

properties not only of self-sensitized TiO2 powders but also of anion-/cation-doped powders. Khomenko et al.13 detected optical transitions and EPR signals in the reduced rutile samples. They assigned the absorption bands 2.3 and 2.9 eV to d−d transitions (i.e., from the localized to the localized states) associated with Ti3+ mainly in interstitial Ti3+ sites. In contrast to localized excitations of Ti3+ Komaguchi et al.11,14 proposed that the visible light excitation of Ti3+ is associated with transitions of localized electrons at Ti3+ sites into the TiO2 conduction band. In ref 15 we reported for the first time on the photocatalytic reduction of NO by CO into N2 and CO2: CO + NO + hν → N2 + CO2 on TiO2−x (Degussa P-25) under UV and vis (λ > 400 nm) light irradiation at room temperature. The remarkable feature of this reaction is that its quantum yield is higher for the sub-bandgap excitation (λ > 400 nm) than for the intrinsic one (365 nm). The photoreaction is characterized by a high selectivity of photoreduction of NO to N2 (90−95%) and a very stable activity of the TiO2−x photocatalyst. It was supposed that the photocatalytic activity of nonstoichiometric TiO2−x in the vis range is associated with localized electron-donor centers (Ti3+ ions and F+ and F-centers), and with the hole centers of Vtype O− localized near the Ti vacancy. The electrons captured in the oxygen vacancies act as donor-like states. The photoactivation of F- and V-type centers and the reactions of activated centers with gas molecules have been thoroughly studied earlier in a number of wide-bandgap oxide insulators using a complex of physical methods.16 We have shown that the spectra of photochemical processes on the surface reproduce the absorption spectra of F- and V-type centers.16 The models developed for wide-bandgap oxide insulators were used in order to describe the processes occurring on a reduced semiconductor TiO2−x.15 Fourier transform infrared (FTIR) and temperatureprogrammed desorption (TPD) studies of the interaction of (1) NO−oxygen mixture and (2) of NO2 with TiO2 was carried out in refs 17 and 18. A variety of adsorbed NOx species has been observed, thus confirming the complexity of the chemistry of nitrogen oxides on the titania surface. In ref 19 the interaction of oxygen molecules with the Degussa P-25 TiO2 surface under UV (λ = 365 nm) and vis (λ = 436 nm) irradiation at T = 293 K was investigated by means of mass spectrometry and the TPD spectroscopy. The UV or vis irradiation in the 18O-enriched oxygen has induced an intensive photostimulated oxygen isotope exchange and equilibration (POIEx and POIEq) via a weakly bonded intermediate O3− due to the interaction of 18O2 with a hole center Os−. The coexistence of two independent excited electronic subsystems, the “fast” one for the excitation in the UV region and the “slow” one for the visible region, was revealed. However, the processes of optical excitation of Ti3+ and F- and V-type centers in TiO2−x and the reactions of the excited centers with gas molecules are rather poorly investigated. Although much has been discussed on the possible role of the Ti3+ and Fand V-type centers in TiO2 photocatalysis, the crucial arguments should be obtained from the analysis of experimental photoactivation spectra. So the direct measurements of photoactivation spectra of the molecular processes over the photoexcited TiO2−x could clarify the nature of the electronic excitation in photoactivated TiO2−x. This work continues the research conducted earlier in a series of studies.15,17−19 In the present work the “photoactivation spectroscopy”16 is used to study the photon-stimulated oxygen adsorption/

2. EXPERIMENTAL SECTION The Degussa P-25 TiO2 powder (specific area, 50 m2/g; m = 40 mg; total surface area, 2 × 104 cm2) was deposited on a wall of a cylindrical quartz reactor (V = 65 cm3) from a bidistilled water suspension, and it was preliminarily purified by heating in a flow of pure oxygen (99.99%) at 0.5 Torr in order to remove the biographic pollutants. The criterion of the sample purity was the absence of CO and CO2 in the output oxygen flow at T = 870 K registered by a mass spectrometer. According to FTIR purified samples contain no adsorbed water. Surface complexes COx and NOx also were not detected. Only very weak bands of hydroxyls were recorded, which do not change during the experiments.17,18 The “reduced” sample was obtained by additional heating in ultrahigh vacuum (better than 10−8 Torr) at T = 870 K for 10−40 min. The “oxidized” sample was obtained by subsequent annealing of reduced samples in oxygen at P = 0.5 Torr at T = 870 K for 1 h and further cooling in O2 with evacuation at 470 K. Special attention is paid to obtain and maintain a clean surface of the sample. Our closed reactor with V = 65 cm3 at 10−8 Torr contains 2.28 × 1010 molecules. With the used sample surface of 2 × 104 cm2 and the sticking coefficient of 1 it can give the coverage of ∼106 molecules/cm2, or ∼10−9 monolayer. At P = 0.5 Torr the reactor contains 1.14 × 1018 molecules, and the number of possible impurities does not exceed 1.14 × 1014 molecules which would limit the pollution coverage to be 3 eV). The lattice oxygen also does not participate in the reactions leading to O 2 desorption.29,32 Lu et al.32 also revealed that the O2 photodesorption exhibits a threshold energy at the TiO2 bandgap of ∼3.1 eV. The third O2 photodesorption component can be assigned to the associative desorption of the TiO2 surface atomic oxygen species desorbing at T ≥ 450 K and also to the structure (lattice) oxygen, which is not revealed in TPD spectra. Indeed we observed a photodesorption under vis irradiation even from samples preheated in vacuum at 723 K without subsequent oxygen adsorption (Figure 4). This confirms that it is the oxygen photodesorption that maintains the concentration of oxygen vacancies in the course of photoactivated reactions. Contrary to our results, Mezhenny et al.33 did not detect any UV-induced defect creation on the (110) rutile surface. It was noted in ref 33 that the used method should have been sensitive to a defect creation “cross-section” of at least 10−24 cm2/photon. The corresponding cross-section obtained in our experiments is about 4 × 10−24 cm2/photon, which is close to Mezhenny’s limit.

Figure 4. Photodesorption of O2 from reduced sample heat-treated in vacuum at 723 K.

Thus, the photodesorption which is considered typical for the UV irradiation,34 in our experiments, it is also observed under the vis illumination. Perhaps this is due to higher sensitivity for the photodesorption detection from powders which reached 10−7 monolayer. It could be caused also by a peculiarity in the physical mechanisms of excitation in the UV and vis regions. Very valuable information on the mechanism of photodesorption excitation certainly could be obtained from the analysis of the photoactivation spectrum of the process. Unfortunately the sensitivity of our setup was not sufficient for such experiments at that time. 3.2. Photostimulated Oxygen Isotope Exchange and Equilibration. The typical isotope exchange kinetic curves in the flow-through mode are given in Figure 5. According to the model of POIEq described in Experimental Section, the equilibration rate is directly proportional to the number of Os−, determined by the centers generation rate and their lifetime. The growth and decrease kinetics are approximated by exponential functions, which allows one to introduce formally the parameters τinc and τdec without examination of the detailed mechanism of the process. These τ parameters represent the lifetimes of Os− centers. The long lifetimes of centers excited in the vis region promotes a high quantum yield of photoreactions in this region. In order to clarify the nature of active centers, a spectral dependence of POIEq was measured (Figure 6). For each λ point, a freshly oxidized sample was used. In Figure 6 we show the stationary exchange rate, Kstat, normalized to light intensity, and the initial exchange rate slope (i.e., value proportional to the hole center generation rate)(dK/dt)ini. Here Iinc is the incident photon flux while Iabs is the absorbed one. An emphasized maximum is well seen in the sub-bandgap absorption region at λ ≈ 420 nm (hν ≈ 2.93 eV) of the spectral dependence of POIEq. The quantum yield of POIE at 420 nm is ∼30 times higher than in the UV region. The obtained spectral dependence is close to the dependence of O− creation measured by EPR in refs 35−37. In contrast to the quantum yield of Os− generation, the quantum efficiency (Figure 6, curve 3) has no pronounced peak in the visible range. This is due to a much stronger absorption in the UV region compensating the decrease of the quantum yield. Additional arguments in favor of the hole nature of active centers were obtained using the CO test molecules which are known to be hole scavengers. During the POIEq measurements the CO molecules were added to the oxygen flow or directly into 21990

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Figure 7. Influence of CO admission on POIEq active centers in the flow-through regime under λ = 436 nm irradiation (Y, the isotopic variable of output O2 flow; Y0, isotopic variable of input O2 flow (thin line); total O2 output flow presented): (a) POIEq on sample preirradiated in vacuum (without CO); (b) the same as in a, but with CO admitted before O2; (c) POIEq postirradiation tail (shown in dashed line) quenched by CO input; (d) no POIEq observed in CO + O2 mixture; (e) sample not poisoned by process “d” (see text for explanation).

of Figure 7c. This is caused not by the CO action but rather by a natural degradation of sample activity to POIEq This degradation is observed in the absence of CO as well, which was demonstrated in special experiments. Though CO2 did not appear in the gaseous phase, it was found in subsequent TPD. Thus, the obtained results confirm the hole nature of Os− active POIEq centers. 3.3. Photoinduced Adsorption of NO and N18O −Ti16O2 Isotope Exchange on TiO2−x/TiO2. It is interesting to compare the spectrum of photoactivated interaction of NO with that of POIEq; the latter is driven by photoactivated hole centers. We have shown earlier6 that the majority of the induced color centers disappear upon NO adsorption, but the color centers responsible for the photocatalysis remain however. As we reported earlier, only a reversible weak NO adsorption on the surface Ti4+ ions occurs in the dark;17,18 the NO coverage in the used pressure range does not exceed 2 × 10−4 monolayer. Upon UV or vis irradiation of TiO2−x/TiO2 in gas NO, the NO pressure decreases considerably, and simultaneously small amounts of N2O and N2 (5−10% of the amount of photoadsorbed NO) appear in the gaseous phase.15,17,18 The NO surface coverage significantly increases (up to 10−2−10−1 monolayer). As it was shown earlier17,18 the number of photoadsorbed NO molecules considerably (by a factor of 6) exceeds the number of the electron-donor sites accumulated during the preliminary UV reduction of TiO2. This result can be expected, taking in mind that the NO molecule exhibits both electron-donor and electron-acceptor behavior. A photoadsorption mechanism was proposed17,18 which includes NO photoadsorption along two parallel routes: (a) via e− capture from the electron-donor center by NO to yield the adsorbed NO− species and (b) via NO oxidizing by the hole O− to give adsorbed nitrite NO2−(ads). A photoadsorption is followed by disproportionation reactions yielding N2O, NO−, NO2−, and NO3− surface species as suggested by FTIR experiments.17,18 However, only the NO2 peak appears in the following TPD spectrum along with the NO peak.17,18 Attempts to detect by mass spectrometry desorption of other products of disproportionation in the gaseous phase fails because of their instability in the gas phase. The TPD spectrum of photoadsorbed NO is shown in Figure 8.

Figure 5. Kinetics of POIE on TiO2−x/TiO2 in flow-through regime. Note the different response times for the UV and vis light and the absence of UV radiation impact on the kinetics of the vis-activated process.

Figure 6. Spectra of POIEq photoactivation under O2 pressure (2.4− 7.5) × 10−5 Torr: (1) POIEq stationary quantum yield; (2) quantum yield of hole centers generation; (3) quantum efficiency of hole centers generation.

the reactor (Figure 7). Active centers are effectively blocked by CO adsorption (Figure 7). The time interval (a) of Figure 7 demonstrates the POIEq caused by a preliminary irradiation in vacuum. It is seen that CO suppresses the POIEq activated by preliminary irradiation in vacuum (b) or in oxygen (c), as well as under irradiation in oxygen (d). However, the effect of CO admission on POIEq is reversible, and POIEq activity is retained after all of these processes (Figure 7e). Note that the response to the illumination during the time interval of Figure 7e is significantly weaker than, e.g., during that 21991

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molecules/cm−2). No products of NO disproportionation either in the gas phase or on the sample surface were found. Therefore, the modeling of the process kinetics is a special problem. Nevertheless, one can evaluate the spectral features at least on a qualitative level. Figure 9 shows that the process has a

Figure 8. TPD spectrum of photoadsorbed NO. The monopeak for an energetically homogeneous surface is shown.

We have not found N2O in the TPD because N2O is weakly bonded to the surface and it can be easily removed upon evacuation at room temperature. Indeed, the weak component at 2240 cm−1 in the IR spectra recorded after NO photoadsorption (the characteristic of ν(NO) in N2O weakly bonded to the surface) disappears after evacuation at room temperature.17,18 The great width of the TPD spectrumfrom 300 to 700 Kis likely due to a variety of the NO adsorption species (individual forms are unresolved) possibly complicated further by the energy inhomogeneity of the surface. According to refs 17 and 18 FTIR defines a set of adsorbed species during NO adsorption on TiO2. The combination of FTIR with TPD will allow one to determine the binding energy of each species. In ref 38 an EPR signal of the radical N2O− was detected on the anchored-to-glass TiO2 UV irradiated at lower temperatures. However, the signal disappeared at 298 K. Courbon and Pichat39 found that under UV irradiation at room temperature the prereduced and preoxidized TiO2 powder (anatase) decomposes NO only into N2O molecules excluding N2 and O2. Rusu and Yates40 observed a slow photoproduction of gas-phase NO at 3.96 eV with the extensive NO depletion at the surface. According to the authors a high cross-section for adsorbed NO depletion and small quantum yields for N2O(g) and NO(g) photoproduction indicate that the dominant photoprocess is the photochemical conversion of the adsorbed NO to the adsorbed N2O. It is supposed that at the first step the NO photodissociation forms an excited nitrogen atom N* which then reacts with NO giving N2O. The evidence for the photoproduction of N2O from chemisorbed NO at vis illumination has been also found−a very low efficiency of N2O production was observed even at 1.8 eV. The authors attributed this effect to electronic excitations from the defect-related filled electronic states in the bandgap region. Also the authors found that a photodecomposition of chemisorbed NO under 326 nm irradiation occurs at depths of the order of 30 μm in the compressed TiO2 powder.40 Note that this fact finds a natural explanation in the Kubelka−Munk model of light scattering by small particles.41 The effect of illumination in vacuo prior to the adsorption of NO (“memory effect”) was tested by manometric method with subsequent TPD measurements. Both methods have not reliably detected any memory effect of NO adsorption on oxidized sample (the obtained value of adsorbed molecules N < 1 × 1010

Figure 9. Spectral characteristics of photoinduced NO adsorption on “oxidized” samples. Kinetics of photoinduced NO adsorption, including its desorption after switching off the light, is shown.

spectral selectivity with an excitation maximum around 420 nm, while under the 365 and 500 nm irradiation the yield of NO photoadsorption is low. Note that this is close to the POIEq activation spectrum. As it can be seen in Figure 9, the photoadsorption is partially reversible; i.e., a part of the NO molecules is desorbed after switching off the light. Unfortunately the lack of sensitivity of FTIR spectroscopy did not allow determination of which of the NO species desorbs.17,18 It is noteworthy to mention that reducing the sample increases the dark consumption by orders of magnitude but does not increase the photoadsorption effect. First-principles calculations42 indicate a large increase of the NO binding energies on the defective surface compared to the case of fully oxidized surface. It emphasizes the role of vacancy defects. Differences in the spectral and kinetic characteristics of the NO photoadsorption for reduced and oxidized samples could be expected, if the main centers of photoactivated reactions would be either the electron centers (typical to the reduced samples) or the hole centers (typical to the oxidized samples). Yet our research of the NO photoadsorption by FTIR spectroscopy17,18 showed a large variety of species adsorbed on both types of centers as (a) via e− capture from the electron-donor center by NO to yield the adsorbed NO− species and (b) via NO oxidizing by the hole O− to give the adsorbed nitrite NO2−(ads). This variety is characteristic for the irradiation both in the UV and in the vis regions. Therefore, under vis irradiation e− and h+ are also generated in pairs. That is possible, if the structures core−shell TiO2−x/TiO2 are formed on the part of surface thus decreasing the bandgap to 2.9 eV. The surface potential in core−shell TiO2−x/TiO2 is the result of superposition of the Schottky barrier in the mesostructure TiO2 and of the near-surface quantum well potential of reduced oxide TiO2−x. The latter potential decreases the Eg value on the surface that explains the vis activity. In near-surface energy quantum well the energy levels are discrete, which explains the long lifetimes of excited states and as a consequence the slow processes, the memory effect in photoadsorption and photoreactions for this oxide.43 21992

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Figure 10. Kinetics of isotopic exchange, accompanying NO photoadsorption under λ > 400 nm irradiation: (a) in flow-through regime (“evac”, NO evacuation; at time = 550 s a fresh portion of NO is admitted); (b) NO isotopic exchange in static regime on the sample preirradiated in vacuum (note that NO photoadsorption is not observed in this case). α kinetics without preirradiation is marked by the asterisk (*). [14 N18O] + [15 N18O] α = 14 16 [ N O] + [14 N18O] + [15 N16O] + [15 N18O]

excitations on the surface under the visible irradiation) and on the spectral data (maximum action is near 420 nm for both electronic and hole centers).

So, both electrons and holes independently take part in NO photoadsorption, each giving its own set of adsorbed NO species, which explains the wide TPD profile. The photoadsorption of NO is accompanied by an intense oxygen isotope exchange (Figure 10). Not only does the oxygen exchange between N18 O and Ti16 O2 occurs under the illumination but also it continues after switching off the light. Moreover the exchange can be activated by the preliminary sample illumination in vacuum (Figure 10b). It indicates the existence of photoactivated long-living exchangeable surface oxygen anions, which are probably those acting in the POIE reaction.

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ASSOCIATED CONTENT

S Supporting Information *

Figures showing XRD and XPS data indicating composition and purity of the sample and its brief discussion and an SEM image of the used sample. This material is available free of charge via the Internet at http://pubs.acs.org.

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4. CONCLUSION Photodesorption of the structural oxygen reduces the surface of the photocatalyst Degussa P-25 TiO2 to TiO2−x thus sensitizing it to the visible region of the spectrum. In order to gain insight deeper into the sensitization nature, a set of model reactions was studied under UV and vis irradiation, i.e., the photoactivated O2 adsorption/desorption and the oxygen isotope equilibration (POIEq) and exchange (POIEx) in the O2−(TiO2−x/TiO2) system, as well as the adsorption and POIEx in the NO− (TiO2−x/TiO2) system. In the previously stated processes, the electron-donor and the hole centers photoactivated in the vis region take part. The hole centers O− are singly active in the O2 photodesorption, in the CO oxidation and also in the POIEq. The hole centers O− as well as the electron-donor centers are active in other studied photoprocesses involving NO and O2 molecules. The maximum of POIEq quantum yield on TiO2−x/TiO2 Degussa P-25 measured in the region of 300−500 nm is around 420 nm which corresponds to the sub-bandgap excitation of TiO2. The quantum yield of the hole centers Os− generation rates was found to have a peak around 420 nm as well. The quantum yield of POIEq in the sub-bandgap absorption region is ∼30 times higher than in the fundamental absorption region of TiO2. The formation of the 2D structures, core−shell TiO2−x/TiO2, on the parts of the surface region and the decreasing of bandgap on these parts are supposed based on the kinetic data (the “fast” excitations in the bulk under UV irradiation and the “slow”

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +7 921 445 4445. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The research was supported by “Physical Methods of Surface Investigation”, “Centre X-ray Diffraction Studies”, and “Nanophotonics” centres of St. Petersburg State University. The work was supported by RFBR (Grant No. 13-03-90426) and by St. Petersburg State University (Grant No. 11.39.1060.2012).

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REFERENCES

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dx.doi.org/10.1021/jp505021a | J. Phys. Chem. C 2014, 118, 21986−21994