Determination of the Energy Levels of Paramagnetic Centers in the

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in the Band Gap of



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Elizaveta Konstantinova, Anton Anurovich Minnekhanov,



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Page 1 of The 28 Journal of Physical Chemistry

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The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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The Journal of Physical Chemistry

Determination of the Energy Levels of Paramagnetic Centers in the Band Gap of Nanostructured Oxide Semiconductors Using EPR Spectroscopy

Elizaveta A. Konstantinova1,2,3*, Anton A. Minnekhanov1,3, Alexander I. Kokorin4, Tatiana V. Sviridova5, Dmitry V. Sviridov5

1

Faculty of Physics, M.V. Lomonosov Moscow State University, Leninskie Gory 1-2, Moscow

119991, Russia. E-mail: [email protected] 2

Department of Nano-, Bio-, Information Technology and Cognitive Science, Moscow Institute

of Physics and Technology, Institutskij 9, Dolgoprudny, Moscow Region 141701, Russia 3

National Research Center Kurchatov Institute, Akademika Kurchatova 1, Moscow 123182,

Russia 4

N. Semenov Institute of Chemical Physics RAS, Kosygina 4, Moscow 119991, Russia

5

Department of Chemistry, Belarusian State University, Nezavisimosti ave. 4, Minsk 220030,

Belarus

Author Information Corresponding author * Faculty of Physics, M.V. Lomonosov Moscow State University, Leninskie Gory, Moscow 119991, Russia. E-mail: [email protected], [email protected], Tel.: +7 495 939-1944


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Abstract A detailed analysis of the nature and photoinduced reactions of paramagnetic centers (PCs) in TiO2 based semiconductor nanoparticles has been performed, and energy diagrams of the investigated samples with the energy level positions in the band gap are determined using electron paramagnetic resonance (EPR) technique under illumination in situ. This study gives a new method for constructing the zone diagram of nanostructured semiconductors. N •, Ti3+, Mo5+ PCs were detected in TiO2/MoO3 samples, and Ti3+ and V4+ centers were observed in the TiO2/MoO3:V2O5 one. The determined energy position of PCs in the band gap of nanostructured semiconductors are located from the valence band on 2.9 eV for Ti3+ ions, 2.7 eV for Mo5+ ions, 2.2 eV for V4+ PCs, and 1.4 eV below the bottom of the conduction band for N • radicals. The effect of illumination is reversible during a long time: approximately 24 hours because of a separation of the photogenerated charge carriers just after excitation between different semiconductor oxide nanosized particles connected by nanoheterojunctions. These results can be useful for understanding the mechanism of photocatalytic reactions and further practical applications of the nanoheterojunction materials such as TiO 2/MoO3 and TiO2/MoO3:V2O5 oxides.


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Introduction The photocatalytic behavior of the wide bandgap semiconductor oxides (e.g. TiO2, MoO3, WO3, etc.) is very sensitive to the structure of energy levels arising in their band gap as the result of bulk doping and surface modification. 1–7 These energy levels are responsible for intraband optical transitions and govern the recombination of the photogenerated charge carriers as well as their involvement into the interfacial reactions. 8–12 The elucidation of the exact position of energy levels which could be involved in the charge transfer reactions and photoelectron trapping is of fundamental importance for the investigations of photocatalytic behavior of TiO 2, MoO3, V2O5 and their nanoheterostructures which are able to accumulate the photoiduced charges and to generate the reactive oxygen species for a long-term activity after illumination.13-15 Nanostructured materials became very attractive during the last decades because of the unique properties such as a quantum size effect and their large surface area which are very important in photocatalysis and photoelectrochemistry. 5,7 Such materials as nanocrystalline titanium dioxide (titania, TiO2) have already found different applications in pharmaceutical industry, solar cells, in indoor and outdoor photocatalytic purification of air and water, and some others.7,16–18 The decrease of particle size results in the increasing the specific surface area which improves the photocatalytic efficiency. A disadvantage of bare TiO2 is that it can absorb only UV light, i.e., the relatively small part of the solar spectrum because of its large band gap between 3 to 3.5 eV. Improving of the light absorption of TiO2 in the visible range is usually associated with its bulk doping or surface modification by incorporating some transition metal ions 9,20, noble metals21,22 or nonmetal elements such as C, N, S or F5–7,11,23,24. However, in many cases, the efficiency of such photocatalysts is rather low. One of the most important characteristics of the doping centers, both in the bulk semiconductor material and on the modified surface, is the exact energy value of the levels produced by dopant in the band gap. Indeed, semiconductors with such centers work as a one with a smaller band gap


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providing light absorbance in a visible range.25–29 Besides this, in the case of mixed nanoheterogeneous oxide semiconductors with wide bandgap, e.g., TiO2/MoO3 or TiO2/V2O5, the injection of photogenerated electrons from TiO2 to the MoO3 or V2O5 nanoparticles results in spatial separation of electrons from holes, which prevents their fast recombination after switching off the illumination.13,14 For the evaluation of energy levels created as the result of doping of a semiconductor matrix or modification of its surface, several experimental techniques were employed including photoluminescence30,31, a photo-Hall effect32, impedance and electrolyte electroreflectance8,33,34, spectroscopic ellipsometry35, diffuse optical reflectance36. These methods where used for bare, metal and N-doped TiO2 as well as for some other nanostructured semiconductors. The main goal of this work is to demonstrate the possibility of determining the photoactive energy levels locating in the band gap of nanoheterostructured oxide semiconductors TiO2/MoO3 and TiO2/MoO3:V2O5 from the EPR spectroscopic measurements under the in situ illumination of the samples. The work includes several connected steps: (i) characterization of the paramagnetic centers (PCs) existing in the samples, (ii) testing the PCs photoactivity, and (iii) measuring the energy transitions and constructing energy diagrams of these semiconductors.

Experimental The nanocrystalline TiO2 in a form of an aqueous sol was prepared via controlled hydrolysis method37 by adding 12.5% NH4OH dropwise to 2.5 M TiCl4 + 0.65 M HCl aqueous solution cooled to 0 °C under vigorous stirring until pH 5 was reached. The obtained precipitate was then thoroughly washed with distilled water and dispersed by ultrasonic treatment. The resultant transparent sol was stable for several days even in the absence of stabilizing agents. The average size of synthesized TiO2 nanoparticles (pure anatase) was of ca. 4 nm as evidenced by TEM. The titania sol was then pulverized over glass substrate heated to 200 C via the procedure routinely used for preparation of thin-film photocatalysts13,14,37 and resultant of nanostructured oxide film


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was then annealed at 450 C. To obtain TiO2 /MoO3 and TiO2/MoO3:V2O5 photocatalytic nanostructures, (NH4)6Mo7O24 or the mixture (NH4)6Mo7O24 + NH4VO3 were added to the titania sol as the precursors of molybdenum and vanadium oxides before pulverization in an amount corresponding to [TiO2]:[MoO3] or [TiO2]:[MoO3 + V2O5] = 5:1 mole ratios. For EPR measurements, nanostructured TiO2, TiO2/MoO3 and TiO2/MoO3:V2O5 films were scraped off the glass substrate and the obtained powders were placed into the quartz-made tubes. All materials used in the work for preparation of TiO2, TiO2/MoO3 and TiO2/MoO3:V2O5 nanostructured oxides were purchased from Sigma–Aldrich Chemical Co. and used as supplied. For preparation of solutions the Milli-Q water was used. The EPR spectra were recorded with a Bruker spectrometer ELEXSYS-E500 (X-band, the sensitivity up to 1010 spin/G). The measurement temperature was varied in the range of 300-10 K with Bruker ER 4112HV temperature control system. The concentration of paramagnetic centers was evaluated using CuCl22H2O monocrystal with known number of spins as the standard. The samples were illuminated directly in the spectrometer cavity in the range of 400-900 nm and by monochromatic light. . As the light source, a halogen lamp was used in the first case, and the 50 W high pressure mercury lamp equipped with the diffraction monochromator was used in the second case. The illumination intensity was of ca. 40 mWcm–2. A flat EPR sample cell of 1 mm thickness was used to provide the most uniform illumination of the samples. The EPR spectra simulation was carried out using EasySpin MATLAB toolbox.38 Optical absorption spectra of TiO2, MoO3 and V2O5 films deposited onto quartz substrates by spraying sols of corresponding oxides were recorded using Shimadzu UV-2550 spectrometer.

Results and Discussion Experimental and calculated EPR spectra of TiO 2/MoO3 sample recorded at 300 and 10 K are shown in Figure 1 and they evidently reflect the existence of superposition of several EPR signals. One can see from Figure 1 that (i) different paramagnetic centers (PCs) are


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observed at different temperatures, and (ii) the relative content of PCs depends strongly on the recording temperature. The simulation of EPR spectra allowed us to determine a spin-Hamiltonian parameters (ĝ and hyperfine splitting, Â tensor values) at reasonable line widths ΔH|| and ΔH. All the calculated parameters are listed in Table 1. These data are necessary for the correct interpretation of the chemical nature of PCs contributing into the EPR spectra.

Figure 1. Experimental (solid black line) and theoretical (short blue dash) EPR spectra of TiO2/MoO3 nanocomposite, measured at 300 (a) and 10 K (b).

The spectrum shape and calculated parameters provide an evidence of the existence of four different PCs in the TiO2/MoO3 sample which are N• radicals, Ti 3+lat and Ti 3+surf ions, locating in the lattice and on the surface respectively, and Mo 5+ centers, the latter being observed only at low temperatures (Figure 1). The values of g||, g, A|| and A obtained in this work are given in Table 1 where the same values known from literature are also listed for comparison. The EPR signal at low magnetic field (B < 355 mT at Figure 1a) belongs to paramagnetic nitrogen atoms N • (nuclear spin I = 1) embedded in the TiO 2 matrix. The N• centers are formed as the result of embedment of nitrogen in TiO2 lattice during sol-gel synthesis in presence of NH 4OH and its further thermal treatment.39–43 Though these ACS Paragon Plus Environment

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signals are usually attributed to nitrogen atoms substituting oxygen atoms in TiO 2 lattice (that results in the formation of

Ti-N•-Ti groups), the possibility of formation of

interstitial nitrogen centers (i.e., the O-N•-Ti groups) has been also discussed.42 The EPR parameters of Ti 3+ centers obtained in this work (Figure 1, Table 1) are in a good correlation with those given in literature.40–44

Table 1. EPR Parameters of the PCs Measured in this Work and Known from Literature. PCs T, K g|| g • a N 300 2.007, 2.0057 2.0043 • 42 a N 77 2.0054, 2.0036 2.0030 • 43 N 77 2.0044, 2.003a 2.002 3+ Ti lat 300 1.971 1.9675 -“10 1.973 1.965 3+ Ti surf 300 1.942 1.928 -“10 1.941 1.930 5+ Mo 10 1.916 1.819 4+ V 20 1.995 1.951 5+ 45 Mo a 300 1.9504 1.9041 45 -“- b 300 1.9450 1.8817 45 -“- c 77 1.9237 1.8755 5+ 46 a Mo 77 1.844, 1.839 1.944 5+ 47 a Mo a 80 1,941, 1,954 1,865 4+ 48 V 1.9825 1.9135 4+ 49 V 77 1.96 1.958 4+ 50 V 80 1.973 1.948 a b c g1, g2; A1, A2; ΔH1, ΔH2 respectively.

A , G 1.3, 3.6b 2.3, 4.4b 2.0, 3.2b 36.5 55 40.38 45.40 46.66 25, 31b 53.3 72.3 50.0 80.0

A||, G 32.9 32.3 32.3 0.5 155.7 66.0 15.4 195.2 153.1 164.9

ΔH, G 2.9, 2.6c 3.9, 2.0c 14.5 ~15 29 ~20 37 29 -

ΔН||, G 2.1 2.8 12 36 ~19 40 17 -

Other lines in the EPR spectra were attributed to the signals from the corresponding transition metal ions with the regard of parameters given in literature44,45 and Refs. therein. Thus, EPR lines observed at low temperature (Figure 1b) with following parameters: g = 1.916, g|| = 1.819, A = 36.5 G, A|| = 0.5 G (Table 1) are rather typical to Mo5+ centers in nanocrystalline MoO 3 and TiO2 doped with molybdenum ions. 44–47 Some variations in calculated parameters can be attributed to the broad anisotropic spectrum lines of Mo5+ centers and different calculation approaches used.


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Figure 2. Experimental (a, c) and calculated (b) EPR spectra of TiO2/MoO3:V2O5 mixed oxide at 30 K. Spectrum c was recorded after 30 min of illumination in situ.

EPR spectrum of the nanocomposite TiO2/MoO3:V2O5 with the molar ratio 5:0.5:0.5, shown in Figure 2, differ considerably from those presented in Figure 1. The initial signal a recorded in the dark conditions is typical for paramagnetic V 4+ or O=V2+ centers described in many publications 44,51,52 and Refs. therein. These centers exhibiting a pronounced tendency to form aggregates. This EPR spectrum has been calculated (Figure 2b), and its spin-Hamiltonian parameters are given in Table 1. Comparing this spectrum with those in Figure 1, one can see that signals from N• radicals, Ti 3+ and Mo5+ ions typical to TiO2/MoO3 sample are not observed in the case of TiO2/MoO3:V2O5 nanocomposite either regarding to a huge intensity of the vanadium signal or as a result of the electron transfer and redox reactions occurring in the nanoheterogeneous mixed oxide system. Computer simulation of the spectrum a in Figure 2 (curve b) evidenced that experimental EPR spectrum results from two types of PCs: the V 4+ centers with g = 1.995, g|| = 1.951, A = 55 G, A|| = 155 G, ΔH = 29 G, ΔН|| = 17 G which manifest themselves as an octet (51V nuclear spin I = 7/2)44,48; and Ti 3+ centers with g = 1.979, ACS Paragon Plus Environment

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g|| = 1.953, ΔH = 28 G, ΔН|| = 35 G.44 The ratio of concentrations of PCs (in at. %) was estimated to be [V4+]/[Ti3+] = 9. It should be noted that our EPR parameters differ from those estimated for VO 2+ centers in similar mixed compounds either in g- or in A-values (Table 1). We assume that this difference can be caused by variations in the local surrounding of the vanadium PCs in the samples prepared by different methods yielding nanophases of specific structure.

Figure 3. EPR spectra of TiO2/MoO3 sample before (a) and after (b) 30 min of illumination at 300 K.

Illumination of the TiO2/MoO3 sample at 300 K with the whole spectrum (with both UV and visible light) for 30 min results in the noticeable changes in line intensities (Figure 3). Indeed, one can see that the amplitude of the EPR signal from N• radicals exhibits a decrease after illumination while the signal from Ti3+surf PCs increases several times. Analogous photosensitivity was observed for Mo 5+ and V4+ centers if EPR spectra were recorded at low temperatures. For example, in the case of V4+ centers, the signal amplitude decreased more than fivefold (Figure 2c). The possibility of modulation of the concentration of N•, Ti3+, Mo5+, and V4+ centers by their selective photoexcitation enables the evaluation of the energy position of the levels corresponding to these PCs in the band gap with the use of the in situ EPR measurements


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(i.e., detecting EPR spectra under illumination with the monochromatic light). Figure 4 represents the wavelength dependencies of the EPR line intensities for PCs forming energy levels in the band gaps of TiO2, MoO3 and V2O5. Defects in TiO 2 based materials play a very important role in the photocatalytic processes including air and water purification.2,5,6,17–20 Therefore, determination of the energy position of the defect levels in the band gap has been carried out. A sample was placed to the EPR cavity, initial (“dark”) EPR spectrum was recorded, all parameters of the spectrometer became fixed, then of the sample started at a certain wavelength , and the EPR spectrum recorded each 10 min. After 30 min of irradiation, we changed a value of  and repeated the procedure. Afterwards, the dependence of the certain EPR line intensity was plotted as a function of the irradiation wavelength (photon energy). Results of our experiments are shown in Figure 4.

Figure 4. The dependence of the EPR signal intensity vs. a wavelength of illumination for different PCs: (A) N• (○), Ti3+ surf ions (●), and (B) Mo5+ (), V4+ (▲).

Figure 4A represents behavior of N• radicals and Ti3+ centers. One can see that in both cases the step-wise changes (jumps) of the EPR signal intensity are observed, i.e. the EPR signal intensity exhibits drastic change at certain wavelength that points to the recharging of PCs. For N• radicals, this jump occurs at hν ≥ 1.4 eV ( = 880 nm) that corresponds to the electron transfer to the conduction band Ec from the energy level located by 1.4 eV below Ec in the band gap. At ACS Paragon Plus Environment

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hν ≥ 2.9 eV ( = 425 nm) the intensity of EPR signal from Ti3+surf centers exhibits an increase that can be explained in terms of electron transfer from the valence band Ev of TiO2 to Ti4+ ions, which are originally diamagnetic. The energy levels corresponding to Ti3+ centers are localized nearby a bottom of the conduction band of TiO2 (so called “Urbach tail”53). Analogous patterns are shown in Figure 4B for Mo5+ and V4+ centers: the EPR signal exhibits an increase for Mo5+ ions under illumination at hν ≥ 2.7 eV ( = 460 nm) and noticeable decrease for V4+ PCs at hν ≥ 2.2 eV ( = 560 nm). The first effect can be explained by the transition of the electron from the valence band of MoO3 nanoparticles to energy levels corresponding to diamagnetic Mo6+ ions which are localized nearby of a bottom of the conduction band of MoO 3. A decrease of the intensity of the EPR signal from V4+ centers at hν ≥ 2.2 eV we explain as a transition of electrons from the valence band of V2O5 to energy levels corresponding to V4+ centers and localized nearby the bottom of the conduction band of V2O5 that yields diamagnetic V5+ centers. We have to note that all samples (discussed above, Fig.4) were divided into two portions: one portion has been illuminated starting from the low photon energy, i.e., from long wavelengths to short ones, while the second portion of the samples was illuminated starting from short to long wavelengths. The step-wise changes (jumps) of the EPR signal intensity are observed for both sets of measurements. Therefore the location of PCs (defect states) in the band gap is the same. It is important that usage of the flat EPR sample cell in our experiments under light irradiation resulted in a reasonably uniform illumination of the samples. If the energy of photons is less than a band gap Eg, only the defect centers are responsible for absorption of light. These centers change their charge state under illumination, either by the transfer of the electron from the energy level of the certain PC to the conduction band or by the electron transition from valence band to the PCs level. We assume that mid-gap centers have rapidly regenerated and this prevents complete bleaching of the EPR signal. As the energy of photons increases and at achievement of fundamental absorption, a pair of photoinduced charge carriers is formed (an electron and a hole), which can also be trapped by PCs


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changing their charge state. We have again observed only partial bleaching of the EPR signal probably due to the regeneration of mid-gap centers (Fig. 4). To obtain the energy diagram of the investigated systems we have measured the band gap energies, Eg, for all the nanostructured oxides under consideration because these values are known to be sensitive to the synthetic routs used being also dependent on the particle size. As an example, several Eg values for different modifications of TiO2 are listed in Table 2. Indeed, Eg values for bulk oxides and small particles are different for the same oxides. Therefore, we have recorded the absorption spectra of TiO2, TiO2/MoO3, and TiO2/MoO3:V2O5 films which revealed straight lines in (h)2 vs. h plots (here h is the energy of incident light and  is the absorption coefficient) that is consistent with the direct optical transitions. The bandgap energies, obtained by the extrapolation of these linear regions to the abscissa axes are collected in Table 2. Table 2. The Eg Values of the Used Metal Oxides. oxide TiO2, nano MoO3, nano V2O5, nano TiO2, anatase, bulk TiO2, rutile, bulk MoO3, bulk V2O5, bulk TiO2, anatase, nano TiO2, anatase, nano TiO2, anatase, bulk TiO2, anatase, nano MoO3, nano TiO2, thin film Ce-TiO2, thin film TiO2, anatase, thin film N-TiO2, thin film

Eg, eV 3.50  0.05 3.01  0.05 2.45  0.05 3.25 3.05 2.75 2.4 3.36 3.21 3.026 2.97 2.85–3.05 3.54 3.07 3.2 2.25

reference This work This work This work 54 54 54 54 55 36 31 31 35 56 56 12 12

The obtained results allowed us constructing the band gap diagrams of our nanostructured materials with the appropriate energy levels of the dopants (defects) within the band gap (Figure 5). ACS Paragon Plus Environment

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Figure 5. Schematic energy diagrams for (A) TiO2 , (B) MoO3, and (C) V2O5 nanooxides.

It should be pointed out that the effect of illuminating the samples was reversible but rather slow: the recovery of the initial EPR signals tales up to 24 h. The latter fact evidences that after switching off light illumination, the photogenerated charge carriers recombine slowly during long time, being, hence, accumulated in mixed oxide nanocomposites. The observed effect can be attributed to a spatial separation of photoinduced charge carriers after excitation in different semiconductor oxide nanoparticles with nanoheterojunctions at their contacts. Such separation prevents fast recombination of charge carriers. This result seems to be of great importance for applications of nanostructured TiO2/MoO3 and TiO2/MoO3:V2O5 photocatlysts for realization of


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various processes of oxidation, purification and desinfection because they remain active for a long time after stopping the illumination.

Conclusions Paramagnetic N•, Ti3+, Mo5+ centers were detected in TiO2/MoO3 mixed oxides, and Ti3+ and V4+ centers in TiO2/MoO3:V2O5 one. For the first time, the energy position of these PCs in the band gap of nanostructured oxide semiconductors was determined using EPR technique with illumination in situ. It has been shown that these energy levels are located above the valence band by 2.9 eV for Ti3+ PCs, by 2.7 eV for Mo5+ centers, by 2.2 eV for V4+ ions, while the energy level corresponding to N• radicals is located by 1.4 eV below the bottom of the conduction band of TiO2. The photoinduced filling of these impurity levels is reversible but complete recovery takes approximately 24 hours due to accumulation of photogenerated charges in the samples. The obtained results provide better understanding in the photocatalyst action mechanism in the case of TiO2/MoO3 and TiO2/MoO3:V2O5 nanoheterojunctions and in their further practical applications.


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Acknowledgements The experiments were performed using the facilities of the Collective Use Center at the Moscow State University. This study was supported by the Russian Foundation for Basic Research (Project No. 16-53-00136-Bеl-а).


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Determination of the Energy Levels of Paramagnetic Centers in the

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