Investigation of Photostimulated Oxygen Isotope Exchange on TiO2

Article pubs.acs.org/JPCC

Investigation of Photostimulated Oxygen Isotope Exchange on TiO2 Degussa P25 Surface upon UV−Vis Irradiation. Ruslan V. Mikhaylov, Andrey A. Lisachenko,* and Victor V. Titov Department of Physics, Saint-Petersburg State University, ul. Ul’yanovskaya, 1, Saint-Petersburg, 198504, Russia ABSTRACT: The interaction of oxygen molecules with TiO2 (Degussa P25) surface under UV (λ = 365 nm) and vis (λ = 436 nm) irradiation at T = 293 K was investigated by means of massspectrometry and thermo-programmed desorption (TPD) spectroscopy. Oxygen chemisorbed on reduced TiO2 surface consists of molecular species O2− and atomic ones. The adsorbed O2 species which are stable at 293 K are not observed on oxidized TiO2. The UV or vis irradiation in 18O enriched oxygen induces an intensive photostimulated oxygen isotope equilibration (POIEq) and exchange (POIEx) via a weakly bonded intermediate O3− due to the interaction of 18O2 with a hole center Os− (which is an exchangeable anion Os2− that captured a photogenerated hole h+). The number of surface exchangeable oxygen anions is ∼2 × 1011 cm−2 for oxidized TiO2 and ∼108 cm−2 for reduced TiO2. There is a reason to consider the observed POIEq as a multiple interaction of Os− with 18O2 molecules (in fact by means of heteroexchange). Thus, the obtained POIEq rate corresponds to the number of Os− centers, while the POIEx rate is proportional to the rate of Os− formation. The kinetics of activation/deactivation of Os− in POIEq was studied in the flow-through mode (at 10−6−10−3 Torr). Under vis irradiation, the hole centers Os− are formed in noticeable quantities and live for hundreds of seconds at T = 293 K, while the UV light transforms only a small part of all possible Os2− into Os−. The lifetime of the latter is short. The lifetime of Os− reduces with the increase of temperature or O2 pressure. Two pathways of Os− deactivation are supposed: the first one includes the recombination of Os− with an electron with an estimated energetic barrier Edes = (0.35 ± 0.06) eV; and the second one is a result of collision of intermediate O3− with gaseous O2.

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INTRODUCTION Titanium dioxide (TiO2) is widely used for photoelectric and photocatalytic conversion of radiation energy in the UV spectral region.1,2 However, only ∼3% of the solar energy belongs to the UV region (λ < 380 nm, hν > 3.2 eV), while the visible light contains more than 40% of the solar radiation. In order to sensitize TiO2 in the visible region, TiO2 is usually doped with transition metals, sulfur, nitrogen, and other elements.1,2 At the same time, the role of intrinsic structural defects in the visible light absorption is not yet clear.3 Earlier we have shown that TiO2 Degussa P25 powder used in practice has the defects responsible for the vis light absorbance.3 The calcinations in vacuum or irradiation in reducing gases (CO, H2) lead to additional reducing of TiO2 Degussa P25 which is accompanied by an enhancement of absorbance bands in the vis range with the maxima at 2.8, 2.5, 2.0, and 1.17 eV.3 On the basis of both optical diffusereflectance spectroscopy (DRS) and UV photoelectron spectroscopy (UPS) results, we have concluded that TiO2‑x centers which are active in the visible region include oxygen anion vacancies filled with one or two electrons (F+ and F centers respectively), accompanied by adjacent Ti3+ ions.3,4 These results allowed us to establish for the first time the photocatalytic reducing of NO by CO into N2 and N2O on TiO2‑x Degussa P-25 under visible light irradiation (λ > 400 nm) at room temperature: CO + NO + hν → N2 + CO2.5,6 It was supposed that electron-donor and hole-donor centers © 2012 American Chemical Society

provide a reduction of NO into N2 and N2O, while hole-type centers only oxidize CO.5 At the same time, the complete analysis of reactions is complicated by emergence of various intermediate products.6 The interaction of oxygen with photoactivated TiO2 is a conventional model reaction for the test of surface species activity. The species formed on TiO2 surface upon UV irradiation in oxygen are widely investigated by means of EPR, and they are mainly O2−, O− and O3−.7−10 It is usually believed that molecular species O2− is a “fingerprint” of reduced TiO2 formed via the electron e‑ transfer from a Ti3+ ion to an oxygen molecule.11 This species is stable on the (110) rutile surface11 and Degussa P-25 powder3 until 410 K. A recent paper12 calls the relationship between the amount of molecular species and the degree of TiO2 reduction in question supposing that the formation of O2− is determined rather by the prehistory of the sample and is related to the bond formation between oxygen and an interstitial Ti ion. Under UV irradiation in oxygen the amount of O2− increases due to the capture of photogenerated electrons by oxygen molecules,7,8,10 while in vacuum the photostimulated desorption (PSD) of O2 is observed at room temperature.13,14 In spite of intensive investigations the role of molecular adsorbed species O2− in Received: June 9, 2012 Revised: September 7, 2012 Published: October 5, 2012 23332

dx.doi.org/10.1021/jp305652p | J. Phys. Chem. C 2012, 116, 23332−23341

The Journal of Physical Chemistry C

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catalysis is still under discussion.2,15 Thus, Formenti et al. had shown by means of EPR that O2− is formed in a broad range of wavelengths up to 520 nm, but it does not correlate with photocatalytic activity of TiO2.7 The creation of trapped holes O− in TiO2,8−10 TiO2/SiO216 was detected by EPR in 110−140 K range under UV irradiation, and a consequent admission of oxygen onto TiO2 led to the appearance of O3− on the surface accompanied by the decrease of O− signal.10 The nature of O− is considered, on one hand, as adsorbed atomic oxygen species Oads− formed during dissociative oxygen adsorption9 or, on the other hand, as oxygen Os− in the TiO2 structure,10,16 resulting from the following reactions: TiO2 + hν (> 3.2 eV) → e− + h+

(1)

Os 2 − + h+ → Os−

(2)

The aim of the present work was to investigate the stability and reactivity of oxygen species on TiO2 (Degussa P25) exposed to light-irradiation in vacuum and in oxygen atmosphere. The photostimulated oxygen adsorption/desorption processes as well as the reactions of oxygen isotope exchange and equilibration over TiO2 upon UV and vis irradiation were analyzed by means of mass-spectrometry and thermo-programmed desorption (TPD).

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EXPERIMENTAL SECTION The Degussa P-25 TiO2 powder (S = 50 m2/g, m = 40 mg, surface area ∼2 × 104 cm2) was deposited on the wall of a cylindrical quartz reactor (V = 65 cm3) from the suspension in bidistilled water and preliminary purified by heating in oxygen flow at the pressure of 0.5 Torr for 10 h at 870 K 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 registered by the mass-spectrometer. The “reduced” sample was obtained by heating in ultrahigh vacuum (better than 10−8 Torr) at T = 870 K for 10−40 min. The “oxidized” sample was obtained by annealing in oxygen at P = 0.5 Torr at T = 800 K for 1 h and subsequent cooling in O2 with evacuation at 470 K. All measurements were performed at 290 K except of specially marked cases. The setup for kinetic mass-spectrometric measurements under static and dynamic conditions and experimental procedures were described elsewhere.23 The mass-spectrometer MI-1201 produced by STANDART factory (Ukraine) (2−600 amu, M/ΔM > 600) was used for the analysis of gaseous phase, for photostimulated desorption and TPD measurements. The design of the reactor allows to perform the measurements in both quasi-static and flow-through regimes. The gas pressure from 10−4 to 1 Torr was measured by a calibrated Pirani-type digital gauge connected to the reactor. The TPD experiments were performed with a constant heating rate of 0.33 K/s. High purity 18O2 and 16O2 were used to prepare equilibrated and nonequilibrated isotope oxygen mixtures. A high pressure Hg arc lamp (SVD-120) equipped with a water IR filter and various sets of standard glass filters (LOMO, Russia), providing the transmission of the emission lines λ = 365, 404, 434, 546, and 578 nm, was used as a light source (Table 1). The photon

At low temperature, the UV induced O− signal remains after switching off the light for several hours (“memory effect”), and disappears when the temperature increases up to ambient temperature due to recombination with electrons released from the traps.8−10 The O− EPR signal is more intensive on oxidized samples than on the reduced TiO2.10 The O3− is a result of interaction of O2 with O−. On the basis of EPR results it was shown that the formed active O3− species has a T-type structure, and the hole h+ in O3− is localized not on the moiety originated from the lattice oxygen, but on the moiety originated from the O2 molecule.16,17 The information about spectral dependences of O− species formation on pure TiO2 surface is poor. In most cases the significant response of TiO2 lies (or is expected to be) in the region of band-to-band transitions (hν > 3.2 eV).10 Nevertheless Volodin et al.8 had shown by means of EPR that the maximum of O− formation on TiO2 powder lies at Ehν = 2.85 eV (λ = 436 nm) that was assigned to the excitation of structure defects with subsequent localization of the hole h+ on the surface traps Os2−, thus forming Os−. One of the methods of investigation of surface activity is the gas−solid oxygen isotope exchange (heteroexchange) and/or equilibration (homoexchange). According to EPR data the UV photostimulated oxygen isotope heteroexchange16 and homoexchange18 on TiO2/SiO2 occurs via an intermediate O3−. Courbon et al.19 have shown that the oxygen isotope exchange over UV irradiated anatase proceeds via a mechanism where only one oxygen atom from the surface participates at a time; i.e., the exchange occurs via an intermediate three-atomic complex: 18

16

O2 +16Os− → O18O16 Os− → O18O + 18Os−

18

Table 1. Photon Fluxes of Transmitted Hg Emission Lines

(3)

λ, nm

combination of glass filters

365 404 436 546 578

UFS6 + BS7 JS10 + PS13 JS11 + SS15 JS16 + PS7 OS13 + ZS7

photon flux (photons/(s·cm2)) (1.0 ± 0.1) (0.63 ± 0.05) (1.0 ± 0.1) (2.7 ± 0.2) (1.6 ± 0.1)

× × × × ×

1015 1015 1015 1015 1015

fluxes of each emission line were measured with a calibrated photocell F-17. All experiments were performed upon vis irradiation with λ = 436 nm except for specially marked cases. POIE measurements were performed in a static regime under the oxygen pressures P = 10−3 − 5 × 10−1 Torr and in a flowthrough regime under P = 10−6 − 10−3 Torr. In the first case, oxygen was admitted at (10−3−0.5) Torr, and the composition and the adsorption/desorption of gas phase were monitored by the Pirani gauge and the mass-spectrometer. In the flow-through mode (providing the measurements in 10−6−10−3 Torr pressure range) the flow of O2 was directed to the reactor connected directly to the mass-spectrometer. In this

There are a few works where the reaction of photostimulated oxygen exchange 3 over TiO2 is compared with its catalytic activity.16,17,19−22 Courbon et al.19 have shown that UV photoinduced oxygen isotopic exchange correlates with photooxidation of isobutene into acetone, thus supposing that the surface hole center O− is responsible for both reactions. Simultaneously Murato et al., by means of EPR, revealed the participation of UV photoformed O3− on TiO2/SiO2 in oxygen exchange and photoepoxidation of alkenes.16,17 Sato20 used the reaction of oxygen isotope equilibration on NOx-doped TiO2 along with the reactions of CO and ethane oxidation in order to demonstrate the photocatalytic activity of yellow-colored TiO2. 23333



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exponential growth is assigned to the structure oxygen desorption which is accompanied by oxygen vacancy formation. TPD spectra after the exposure of reduced TiO2 sample to oxygen (0.07 Torr, for 15 and 30 min) at 290 K are shown in Figure 1 (curves 1 and 2, correspondingly).

case, the deviation of mass-spectrometer output signal from its equilibrium is directly proportional to the change in gaseous phase ((dNgas)/(dt)). The gas pressure P in the flow-through mode was calculated by the formula P = ((L τevac)/V)kBT, where L is the oxygen flow, τevac is the characteristic time of oxygen evacuation (here τevac = 5 s), V is the reactor volume, kB is the Boltzmann’s constant, and T is the temperature. The parameters of the isotope oxygen mixture are as follows:24,25 C34 + C36 (4) 2 18 that indicates the O fraction in the mixture, and a deviation from the equilibrium value α=

Y = 2α(1 − α) − C34

(5) 16

16

18

18

where C32, C34, C36 are the fractions of O2, O O, and O2, respectively. The mixture with Y ≠ 0 is called “nonequilibrated”. Three types of oxygen exchange can occur on the surface:24,25 Homoexchange (type I, having the rate R0 or R for further simplification) without participation of surface atoms; Simple heteroexchange (type II, R1) involves one surface atom at a time, and multiple heteroexchange (Type III, R2), involving two surface oxygen atoms at a time. The total equation describing the isotope exchange reads as follows:24,25 Ng

⎛ dα ⎞ 2 dY = −RY + φ⎜Ng ⎟ ⎝ dt ⎠ dt

Figure 1. TPD spectra: after chemisorption of O2 for 15 min (1) and 30 min (2); after chemisorption of O2 for 15 min and subsequent vis irradiation in O2 for 15 min (3); after chemisorption of O2 for 15 min and subsequent vis light induced photodesorption in vacuum (4).

The TPD spectrum of oxygen adsorbed on reduced P25 sample consists of several overlapping peaks in the 400−650 K region, one of them at 440 K being a characteristic peak of a reduced sample. It consists most likely of two peaks with maxima at 410 K and ∼440 K, both having the molecular O2− nature. The maximal value of oxygen coverage on reduced sample is ∼2 × 1011 molecules/cm2 (∼10−4 monolayer). Here the molecule-surface bond energy Edesorption does not exceed 1.1 eV.3,11 The bond is provided by an electron transfer from electron-donor centers (Ti3+ ions and F+ and F centers) to O2(ads). The nature of other peaks in 450−600 K region will be discussed below. The isotopic composition of TPD spectrum after exposure of reduced TiO2 to nonequilibrated oxygen (αg = 0.46, Y = 0.26) is shown in Figure 2.

(6)

Here φ = ((8R1)/((R1 + 2R2) )), and Ng is the number of gaseous molecules in the reactor volume. If there are no exchanges of types II and III, or both R1 and R2 are much less than R and the number of active sites is much less than Ng, then the eq 6 can be simplified:25 2

Ng

dY = −RY dt

(7)

In case of the flow-through regime the equation describing the homoexchange reads

Y −Y dY = −KY + 0 dt τevac

(8)

where Y0 is the parameter Y of the input mixture and K = (R/ (Ng)) is the equilibration rate constant. The key parameter of homoexchange is the absolute rate of reaction R = KNg = KLτevac

(9)

where L is the oxygen flow. In the present work both types of isotope oxygen mixture equilibrated (αg = 0.73, Y = 0) and nonequilibrated (αg = 0.44− 0.46, Y = 0.26−0.29))were used.

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RESULTS AND DISCUSSION Photostimulated Oxygen Adsorption/Desorption Processes. After exposure of an oxidized TiO2 sample to oxygen (0.07 Torr, for 30 min) at room temperature the sample does not demonstrate any peaks in subsequent TPD spectrum in 300−870 K range except the exponential growth at the temperatures above 850 K; i.e., no stable adsorbed oxygen species are formed on the oxidized TiO2 surface. The

Figure 2. The typical isotope composition of TPD spectrum after exposure of reduced TiO2 to oxygen mixture 16O2 + 18O2 (αg = 0.46, Y = 0.26, P = 0.07 Torr for 30 min) at 290 K. Inset: parameters α and Y of TPD spectrum. 23334



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The concentration of 18O denoted by α and the parameter Y remain constant in the temperature region 400−450 K (Figure 2, inset) and then reduce at temperatures higher than 450 K reaching their local minima (α = αmin, Y = Ymin) at 550 K. The dependences α and Y vs T are insensitive on the initial nonequilibrated mixture composition (i.e., αg and Yg ≠ 0): they both have a plateau in the 350−440 K region with α = αg and Y = Yg (the molecular form O2−) and decreasing branches with local minima at 550 K. The same isotopic composition was obtained by Yanagisawa et al.14 The presence of two peaks at 410 K and ∼440 K is seen in Figure 3, and they are probably

The TPD of oxygen after PSD is presented in Figure 1, curve 4. The isotopic components (m/e = 32, 34, 36 amu) of TPD of chemisorbed oxygen and those after oxygen PSD are shown in Figure 3, and the corresponding difference of total spectra is shown in Figure 4 (curve 1).

Figure 4. Changes in chemosorbed oxygen TPD spectra caused by subsequent: (1) O2 photodesorption; (2) O2 photoadsorption.

The photodesorption causes the changes in TPD spectra only in O2− molecular peaks at 410 and 440 K, which in their turn become apparent in the difference spectra (Figure 4). At the same time PSD does not affect the atomic species (Figure 4, curve 1). The photodesorption of nonexchanged oxygen molecular species from the (110) rutile surface was also observed by T.L.Tompson et al.27 (110) at 180 K and by Yanagisawa14 at room temperature. The photodesorption of oxygen is accompanied by the formation of surface oxygen vacancies. We also detected the PSD of oxygen from “oxidized” TiO2 under vis irradiation without preliminary O2 exposure (not shown here). The vis irradiation of a sample in oxygen leads to the increase in TPD spectrum in 400−600 K region (molecular and atomic species) (Figure 1, curve 3 and Figure 4, curve 2). The growth of molecular species coverage is due to the capture of photogenerated electrons by O2 with the formation of O2−.7,8,10 Concerning the increase of atomic species coverage, it is probably due to the oxygen vacancies formation with their subsequent filling, thus enhancing the contribution of associative desorption in TPD. The dissociation of oxygen on TiO2 (110) under UV irradiation at 28 K was found by N. G. Petrik and G. A. Kimmel, and it was supposed that the dissociation is result of reaction of O22− with an electron.26 Photostimulated Oxygen Isotope Exchange (POIEx) and Equilibration (POIEq). Quasi-Static Regime. The vis (or UV) irradiation of the reduced sample in nonequilibrated oxygen (Y0 = 0.27) causes the oxygen isotope equilibration (i.e., Y falls down to 0) within a few tens of seconds even at relatively high pressure, P = 0.1−0.5 Torr, indicating a high rate of homoexchange. No change of α was observed, and we will show below that it is due to a low quantity of surface exchangeable oxygen rather than due to the absence of heteroexchange. Unfortunately, no kinetic features of Y behavior could be revealed because of the high equilibration rate R. The sample preirradiated in vacuum with vis light showed significant activity in equilibration after addition of the mixture indicating the formation of long-lived active centers. An irradiation of the mixture of nonequilibrated oxygen and CO was not accompanied by the POIEq reaction, in accord with

Figure 3. Isotopic composition of oxygen TPD after chemisorption of nonequilibrated oxygen mixture (lines) and chemisorption with subsequent PSD (circles).

caused by two different types of molecular species O2− on TiO2. The existence of few adsorption species O2− on TiO2 P25 surface after reducing at temperatures higher than 600 K was obtained by Attwood et al.9 that supports our results. The desorption of oxygen in 450−600 K region is accompanied by the isotopic exchange (Figure. 2, inset). Henderson et al.11 have shown that at room temperature the first stage of oxygen adsorption on reduced TiO2 surface is a vacancy filling, i.e. a dissociative adsorption; and the second stage is the formation of O2− species with the ratio of 3 molecules per one defect. Also the dissociative mechanism of oxygen adsorption at low pressures (10−7 − 10−5 Torr) was obtained in the work of Formenti et al.13 It is possible that the peaks that we observed in 450−600 K region are due to the desorption of such atomic species. The accumulated material on chemisorbed oxygen reveals the following feature of this region: in most cases the intensity of the total peak at 500 K looks like a “satellite” of the molecular peak at 420 K and independently on the initial coverage its intensity is 1.7−2 times less than that for the 420 K peak. Such a behavior indicates the relation between the chemisorbed molecular and atomic species which could be similar to that suggested by Henderson.11 The decrease of α and Y values of desorbing oxygen in the 450−600 K region (Figure 2, inset) indicates that the desorption is accompanied by oxygen rearrangement near the filled vacancy. The vis light irradiation of TiO2 in vacuum causes a desorption of chemisorbed O2. The O2 desorption kinetic is similar to that obtained in works.13,14,26 The isotopic parameter α of photodesorbing oxygen is that of the preliminary adsorbed oxygen, showing the absence of heteroexchange, while Y decreases with the time of irradiation, i.e. the photodesorption is accompanied by the isotope equilibration. 23335



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results reported by others22 who assigned the absence of oxygen exchange to the blocking of Os− holes by CO. There are no significant changes in isotopic composition of TPD spectrum after POIEq (Y0 = 0.27, time of irradiation 2 min) in comparison with the TPD after exposure to nonequilibrated O2 mixture. The TPD spectrum and the behavior of both α and Y are similar to those shown in Figure 2. The desorbing molecular species having the common TPD maximum at 440 K have the value Y440 K = 0.27 in spite of the final value of Ygas = 0. The increase of irradiation exposure in oxygen mixture up to 15 min leads to a small reducing of Y value for the maximum at 440 K. In order to establish the role of molecular species in the POIEq process the sample was exposed to 18O2 (α0 = 0.73) atmosphere in the dark (15 min, at pressure 0.07 Torr) in order to form the adsorbed 18O2− (α440 K = 0.73) species and then it was irradiated in 16O2 under the same conditions. If the molecular form O2− participates in homoexchange processes then the final composition of molecular form at 420 K should consist of predominantly 16O2− (i.e., the final α440 K should be close to 0) because the number of gaseous molecules 16O2 exceeds at least by 2 orders of magnitude the number of chemisorbed 18O2− species, and the irradiation time is more than sufficient for to reach a saturation value (15 min against few seconds). On the contrary, the irradiation of sample containing 18O2− species in 16O2 for 15 min shows the partial substitution of molecular forms at 440 K which are combined, in the case of incompletely saturated surface, with addition to them, i.e. α440 K reduces to a nonzero value while the parameter Y440 K increases from 0 indicating that the substitution occurs without intermolecular exchange by atoms and can be written as follow:

Figure 5. Typical isotope composition of output O2 flow, isotopic parameters and POIE rate constant K upon vis (λ = 436 nm) and UV (λ = 365 nm) irradiation of reduced TiO2 in nonequilibrated oxygen mixture.

O2(ads)− + h+ → 18O2(gas)

(12)

O2(gas) + e− → 16O2(ads)−

(13)

The POIEq in the flow-through regime over the reduced TiO2 sample is characterized by the rate R increase to its stationary value R0 after the beginning of vis irradiation within 100−150 s (Figure 5). It is accompanied by a weak heteroexchange (POIEx): α rapidly drops and then it tends to its initial value α0. After switching off the light the equilibration continues (“memory effect”)R slowly reduces down to zero, while the heteroexchange ceases immediately. An effectiveness of POIEq, i.e. a ratio of POIEq rate R0 to the light intensity Iphotons((R0)/(Iphotons)) × 100%, is quite high and reaches the values of 60%. The both POIEq and POIEx cease after admission of CO in the oxygen mixture. The similar behavior of POIEq and POIEx is observed on the oxidized sample, but their values many times exceed the rates on the reduced sample. The increase/decrease times of POIEq strongly depend on the spectral region of irradiation and on the oxygen pressure. In the vis region these times are long, while the UV irradiation causes sharp fronts (see Figure 5 and 6). The increase of the pressure causes the reduction of these times.

The photogenerated hole reacts with the adsorbed molecular oxygen resulting in molecule desorption, whereas the photogenerated electron causes the gaseous oxygen adsorption. The ratio of the rates of the processes (12) and (13) depending on the TiO2 state determine the shift of the balance toward to substitution or addition. An additional argument rejecting the participation of O2− in POIEq is that the highest POIEq rate is observed on the oxidized sample, which is unable to form the molecular species O2−. Thus, the observed photostimulated oxygen isotope equilibration does not occur via the molecular form O2− that coincides with the result of Tanaka.28 Taking into account the high rate of the observed POIEq one concludes that it occurs via the weakly bonded surface species, which are not observed in TPD in the 300−870 K region. Flow-Through Regime. The typical kinetic curves of isotope exchange in flow-through mode are presented in Figure 5.

Figure 6. Efficiency (E) of oxygen isotope equilibration on oxidized TiO2 upon UV and various vis irradiations. The inset displays that vis POIEq and UV POIEq are additive.

O2(ads)− + 16O2(gas) + hν → 16O2(ads)− + 18O2(gas)

18

(10)

−

The degree of O2 substitution (characterized by the final α and Y values) depends on the irradiation exposition. Thus, the total mechanism describing the substitution under irradiation can be presented as (for instance without consideration of the nature of excitations in the vis region): TiO2 + hν → e− + h+ 18

16

(11)

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Os− interacts with gaseous molecules 18O2 forming a shortliving intermediate three-atomic complex O3− during the lifetime of which the heteroexchange occurs (eq 15a or 15b). The hole center Os− is able to form an ozonide complex O3− many times resulting in observed equilibration before its deactivation. After switching the light off the formation of new hole centers Os− ceases (the POIEx ceases), and further POIEq effect occurs via the hole centers Os− accumulated up to the moment of switching off the light, which number reduces with time due to the deactivation. Thus, the heteroexchange kinetics measured in flow-through regime demonstrates the rate of Os− formation, whereas the homoexchange indicates the current number of surface Os−. The results of irradiation of the sample in various spectral regions are shown in Figure 6. One can see that POIEq occurs even under irradiation with λ = 578 and 546 nm. The nature of these excitations is out of the scope of this work, and further investigations are required. There is a surprising fact that the exchange rate under irradiation in the region of TiO2 fundamental absorption is not so high compared with the rate of the exchange induced by 436 or 404 nm lines. The low rate under UV light can be explained, on one hand, by intensive Os− recombination processes, and on the other hand, by a small size of the active part of the sample (spatially nonuniform) due to a strong UV light absorption, or by both effects simultaneously. If only the first assumption is valid and a uniform UV exposure of the sample occurs, then the UV light must strongly reduce the effects induced by vis light due to a high rate of recombination of hole centers Os− with UV generated electrons. However (see Figure 6, inset) the irradiation with λ = 365 nm line in time span of the postirradiation exchange tail, caused by previous λ = 436 nm irradiation, results just in a superposition of both effects (the UV stimulated exchange appears on the background of postirradiation tail) without any UV light influence on the tail. This demonstrates rather a spatial independence of vis and UV effects. Moreover the number of 18O consumed by the UV exposed sample in heteroexchange is much less than that of vis exposed, confirming once again the different surface areas available for activation. Special experiments were performed in order to estimate the ratio of activated surfaces under UV and vis irradiation. The oxidized sample was vis irradiated in 18O2 flow during 70 min in order to equalize αsurf and α0, and then in 16O2 flow upon UV light to extract all the possible 18O that required 6−7 min. The number of extracted 18O atoms under UV does not exceed 10− 15% of the number consumed by the sample upon vis light. Thus, we can suppose that the activated surfaces in cases of UV and vis irradiation differ significantly, and the activated exchangeable oxygens are the same in both cases. Consequently the strong difference in both exchange rates and their character in UV and vis regions can be explained by a combination of two factors: different O s − recombination rates and spatial heterogeneity of the parts of the sample participating in the exchange. Apparently under vis irradiation the whole sample surface is active, whereas under UV irradiation just a limited part of the sample participates in the exchange because of a relatively small depth of the UV light penetration. The intensive photogeneration of electron−hole pairs under UV light leads to the sharp raise of POIE, but at the same time causes the rapid Os− recombination due to their interaction with electrons that provides short postirradiation POIEq tail. Under vis irradiation

The equation describing the change of α in a gaseous output flow as a result of heteroexchange with surface, having its αsurf, reads: α −α dα = Kα(αsurf − α) + 0 τevac dt

(14)

where Kα is the POIEx rate constant, and α0 is the α of input oxygen flow. Thus, the fast tendency of α of the output mixture flow to α0 during irradiation (Figure 5) means that there is a limited number of exchangeable structure oxygen on the surface, and αsurf = ((N(18Oexch))/(Oexch)) rapidly becomes equal to α0. From vis induced heteroexchange kinetics the maximal numbers of surface exchangeable oxygen were calculated: they are about 108 atoms/cm2 (or less) for reduced sample, and 2 × 1011 atom/cm2 for oxidized sample. Over the reduced sample, the total exchange occurred within a few minutes, while the oxidized sample required several hours. The maximal value of surface exchangeable oxygen comes out of the oxidized sample, whereas the sample reduction removes the exchangeable oxygen atoms together with other nonexchangeable ones, thus reducing their amount. The estimation of the number of exchangeable oxygen atoms per TiO2 Degussa oxidized particle gives the value of ∼5 atoms/particle, assuming that the mean particle size is ∼20 nm. This estimation is close to the number of UV induced O− (10 sites per particle) obtained by Berger10 by EPR measurements for particle size of 13 nm. In order to establish the stability of surface exchangeable oxygen towards the diffusion into bulk the oxidized sample was vis irradiated in 18O2 within 3 h to reach the equality of αsurf and α0, and then in 16O2 within 1.5 h in order to extract all the possible 18O. The amount of the extracted 18O is 45% of the amount previously injected to the sample. The impossibility to gather all isotopes is probably related to the substitution of surface 18O by bulk 16O. Nevertheless, such estimation allows us to suppose that the diffusion of active oxygen can be neglected in kinetic description because the usual times of experiments on POIE do not exceed 1 h even in cases of slow processes. The obtained values of POIEq rates exceed by several orders the heteroexchange ones. The heteroexchange immediately ceases after the light switching off whereas the homoexchange slowly decreases for a long time indicating the presence of longliving active centers. Moreover the both processes cease after admission of CO in the mixture. All facts lead to the supposing that both homo- and heteroexchange can be considered as interrelated processes with a common nature and described by the following schemes: O2 + 16Os− → 18O18O16Os− → 16O18O + 18Os−

(15a)

O2 + 18Os− → 16O16O18Os− → 16O18O + 16Os−

(15b)

18 16

where Os− is an activated surface exchangeable oxygen (hole center), and O2 is a gaseous oxygen molecule. The eqs 15a and 15b describe the steps of heteroexchange, while the consequence of eqs 15a and 15b describes the homoexchange: 18

O2 + 16O2 → 16O18O + 16O18O

(16)

Thus, the overall picture can be described in the framework of the following model. Upon irradiation the surface exchangeable oxygen Os2− captures the photogenerated hole h+ and turns into a long-living hole center Os−. This hole center 23337



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the efficiency of both electron and hole formation is low and can be accompanied by their localization in the bulk or/and in surface traps that determines the slowly increasing branch of the POIEq rate. But, on the other hand, a low quantity of free electrons leads to a low O−s recombination. The POIEq under Vis Irradiation. The POIEq under vis irradiation is of special interest, thus all the results presented below are obtained under λ = 436 nm irradiation. The line λ = 436 nm (2.83 eV) is close to the maximum of the absorption band at 2.8 eV assigned to absorption of oxygen vacancies.3 On the other hand, according to the results of Volodin et al.,8 the spectral maximum of O− formation is at ∼2.8 eV. The process F+ + O2 − + hν(436nm) → F + O−

(17)

can be considered as the primary step of photoinduced POIEq. In reaction 17, an electron of the valence band transfers onto the lowest empty level of an excited F+ center, thus forming at once an electron-donor F center and a hole O−. The latter can further be localized on the surface exchangeable oxygen thus forming Os−. The deactivation of Os− can be presented as follows: as a result of the interaction with an electron e‑ released from the trap upon irradiation or in the dark

Os− + e− → Os 2 −

Figure 7. Dependence of stationary POIEq rate R0 on oxygen impingement rate ν (or pressure P).

The factor 1/2 in eq 22 is introduced because the ratio ((n0)/ (Rmax)) gives the time of one homoexchange act, which in turn consists of two acts of heteroexchange, i.e., of the consecutive formation of two three-atomic complexes on the same Os− center. Thus, the lifetime of O3− does not exceed 5 ms. The characteristic times τincr and τdec in eq 20 and 21 were the main parameters of the processing. The pressure dependences of τincr and τdec are given in Figure 8.

(18)

or as result of a delocalization of the surface hole Os− with a subsequent recombination with an electron coming from the bulk or the surface F-center: O− + [F···O2−] + kT → O2 − + [F+···O2−]

(19)

The increasing and decreasing branches of vis induced POIEq kinetics (Figure 5) in the flow-through mode can be fitted with exponential functions: ⎛ ⎛ t ⎞⎞ R incr = R 0⎜⎜1 − exp⎜ − ⎟⎟⎟ ⎝ τincr ⎠⎠ ⎝

(20)

and ⎛ t ⎞ Rdecr = R 0 exp⎜ − ⎟ ⎝ τdecr ⎠

Figure 8. τincr and τdec dependences on oxygen pressure. Inset: The dependence of the decay time POIE in coordinates ln((1/(τdecr)) − cqP(1 − exp(−cPP))) vs 1/kT.

(21)

correspondingly. It was established that the values of R0, τincr, and τdecr depend on oxygen pressure P, temperature T and light intensity. With the light intensity change from 3 × 1013 to 1 × 1015 photons/(s cm2) the value R0 increases monotonically while the time τincr decreases. The dependence of stationary POIEq rate R0 (Figure 7) on oxygen impingement rate ν (proportional to pressure) is described by a linear function R = aν (where a = (5.3 ± 0.4) × 10−8), supporting the model of exchange by means of collisional interaction via a three-atomic intermediate complex O3−. In this case the slope a is related with the O s− concentration n0 by the expression a = σn0, where σ is the cross section of O2 interaction with Os−. Knowing the maximal value of n0 ∼108 cm−2 for the presented result, the value of σ can be estimated to be of the order of ∼10−16 cm2 which is close to the collisional cross section of oxygen. For the known value n0 ∼ 108 cm−2 and the maximum obtained homoexchange rate Rmax ∼ 1010 molec s−1 cm−2 the upper limit of the intermediate O3− lifetime τmax O3− can be estimated as follows: n 0 − τOmax < ∼ 0.005 s 3 2R max (22)

Both dependences are characterized by a plateau in the lowpressure region (up to 2 × 10−5 Torr) and a subsequent decaying with the pressure increase. These dependences explain the reason for the absence of POIEq “memory effect” at higher pressure of oxygen. The reduce of characteristic times with pressure indicates that the quenching of surface hole centers Os− occurs and it is due to interaction of these centers with the gaseous oxygen. At the same time the plateau in the lowpressure region allows to suppose rather a “O−−gas” interaction than the interaction “Os−−gas”. This assumption is based on the fact that the probability of the interaction of O3− during its lifetime ( 10−4 Torr becomes comparable with the 23338



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lifetime of O3−. Thus, as the pressure increases, the interaction probability grows leading to the collisional deactivation of the surface hole centers Os− according to the scheme: O3− + O2(gas) → Os 2 − + 2O2(gas) + h+(Obulk −)

τdecr are connected with the rate constants ki and kr in following way:

ki

1 kr

(34)

(35)

is the Os− recombination rate due to O3− + O2(gas) collision

The constant kq(P) constant (pressure dependent) with the subsequent Os− quenching. The contribution of the collisional deactivation of the centers can be written as follows: kq(P) = cqP(1 − exp(−cPP))

(36)

O3−

where cq is the coefficient including both the complex formation parameter and the cross-section of collision of gas molecules with O3−; P is the O2 pressure; the factor (1 − exp(−cPP)) shows the probability of the O3− + O2(gas) interaction during the lifetime of O3− complex, where cp is the coefficient of gas molecules interaction with O3− complex including its lifetime and the cross section of interaction. The eqs 33 and 34 can be rewritten using the formulas (35) and (36):

kf

Os− + O2(gas) → O3(surf )−

(25)

kd

O3(surf )− → Os− + O2(gas)

(26)

kq

O3(surf )− + O2(gas) → Os 2 − + 2O2(gas) + h+(bulk) kdes

Os− + e− ⎯→ ⎯ Os 2 −

(27) (28)

Os−

concentration n is:

τincr =

(29)

τdecr =

dn = ki(n0 − n) − krn dt

where Os−,

dn = − k rn dt

(33a)

1 kdes + cqP(1 − exp(−cPP))

(34a)

Table 2. Parameters of the Fitting Dependences of τincr and τdecr on Oxygen Pressure

ki (s−1) kdes (s−1) cq(s−1 Torr−1) cp (torr−1)

(30)

The solution of eq 29 is n0 n(t ) = (1 − exp(− (ki + kr )t )) 1 + kr /ki

1 ki + kdes + cqP(1 − exp(−cPP))

Thus, in the framework of the supposed model the dependences τincr and τdecr on pressure can be described by four parameters ki, kdes, cq, and cp using the eqs 33a and 34a. The results of fitting of these dependences are presented in Figure 8, and the values of coefficients are given in Table 2.

n is the number of n0 the maximum number of 8 −2 exchangeable surface oxygen atoms O2− for s (∼10 cm 11 −2 the reduced sample, ∼ 10 cm for the oxidized sample); ki is the rate constant of Os− generation (including light intensity, light absorption coefficients and carrier transport); kr is the rate constant of Os− recombination. In the absence of illumination the eq 29 is simplified:

(31)

from the dependences τincr(P)

from the dependences τdecr(P)

0.0027 0.0013 146 1537

0 0.0013 125 4183

The decay time τdecr measured at constant pressure reduces with the temperature increase in the range 290−390 K indicating the presence of an energetic barrier in O s − deactivation process described by the eqs 27 and 28. It is not easy to reveal the nature of the barrier. Nevertheless if one assumes that no drastic changes occur in the quenching mechanism via O3− + O2(gas) collision in the 290−390 K range (eq 27), this barrier can be considered like the Os− to Os2− transition one due to the recombination of a hole center with an electron released from the trap (eq 28). On the basis of this

with the initial condition n(0) = 0, and the solution of eq 30 is n(t ) = nst exp( −krt )

τdecr =

kr = kdes + kq(P)

(24)

The rate of the change of the surface

(33)

Taking into account that τincr and τdecr depend on the pressure (Figure 8), the rate constant of centers recombination kr can be expressed as the sum of constants of deactivation (kdes), (according to eqs 18 and 19) and of the collisional quenching (kq(P)) (eq 27):

(11)

Os 2 − + h+ → Os−

1 ki + kr

(23)

The deactivation may be accompanied by a diffusion of the hole into the bulk with its subsequent recombination. The measurements performed at elevated temperatures showed that the POIEq rate R0 decreases with the temperature, and no POIEq was observed at T > 420 K. The increase and the decrease times τincr and τdec reduce with the temperature, and at T > 390 K their values become indeterminable. After the sample cooling the POIEq recommences. The reduction of the time of posteffect tail with T is related to the enhancement of O−s deactivation. The overall processes can be presented by the following sequence of stages: TiO2 + hν → e− + h+

τincr =

(32)

Os−

where nst is the number of at the moment when the light is switched off. Since the POIEq rate R(t) is assumed to be proportional to n(t), the eq 20 and 21 describing the increasing and decreasing of the POIEq rate R(t) can be rewritten in the form 31 and 32 respectively. The obtained increase and decrease times τincr and 23339



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TiO2 + hν → e− + h+

assumption, the rate constant kdes of eq 28 can be written as follows: 0 kdes = kdes

⎛ E ⎞ exp⎜ − des ⎟ ⎝ kT ⎠

ki

Os 2 − + h+ → Os−

■

CONCLUSIONS (1) The chemisorbed oxygen on the reduced TiO2 (Degussa P25) surface appears in the subsequent TPD as a set of overlapped peaks in the 380−600 K region, where, according to isotopic composition measurements, two peaks (410 and 440 K) belong to the molecular species O2−, and other peaks (in the region 450−500 K) have the associative character involving the lattice oxygen atoms. No TPD peaks are observed in this region on oxidized TiO2, but at a temperature above 850 K, it appears as intensive exponential O2 growth due to the desorption of structural oxygen accompanied by vacancy formation. The UV or vis irradiation of reduced TiO2 in oxygen (P = 0.1−0.3 Torr) entails a photoadsorption, thus increasing the dark oxygen coverage by one-third. The UV or vis irradiation of TiO2 containing the preliminary adsorbed oxygen causes the desorption of molecular species, without affecting the associative forms. At the same time the desorption of small amounts of structural oxygen was registered. (2) The UV or vis irradiation of TiO2 in 18O isotope enriched oxygen is accompanied by both photostimulated hetero- and homoexchange caused by photoactivated surface exchangeable oxygen Os− (surface hole center). The heteroexchange occurs via the formation of short-living intermediate O3− according to the scheme O2 + 16Os− → 18O18O16Os− → 16O18O + 18Os−

(24)

Under vis (λ = 436 nm) irradiation the hole is the result of exciting of an oxygen vacancy (F+-centers). The Os− centers being formed occur to be long-living (a few hundreds of seconds at room temperature) due to a low recombination with electrons which photogeneration under λ = 436 nm is low too. It explains the slow POIEq rate increasing after the beginning of vis irradiation, as well as the observed postirradiation memory effect. Os− species formed upon UV (interband transitions) readily recombine with photogenerated electrons, thus forming the sharp fronts of increase/decrease of POIEq rate. Two processes of vis induced Os− deactivation are supposed: the recombination of surface hole center Os− with electron released from the trap and the collision quenching (an interaction of gaseous O2 with O3− resulting in subsequent destruction of the intermediate and deactivation of Os−). The first one (recombination process) has the deactivation energy barrier Edes = 0.35 ± 0.06 eV.

(37)

Combining the eqs 34a and 37 and taking into account the values presented in Table 1 we can write down the temperature dependence of the decay time τdecr as a function ln((1/(τdecr)) − cqP(1 − exp(−cPP))) vs 1/kT at constant pressure P (Figure 8, inset). The slope of this dependence gives the value of the deactivation energy Edes = (0.35 ± 0.06) eV and the value k0des ∼ 3000 s−1. The obtained Edes and k0des values allow to simulate the surface concentration of active sites n(t) in vacuum at 300 K using eqs 32 and 35 and supposing kq(P) = 0. The calculated curve n(t) (with Edes = 0.375 eV and k0des = 3000 s−1) is indeed close to the experimental curve R(t) measured at lowest used pressure P = 4.7 × 10−6 Torr.

18

(11)

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■ ■

ACKNOWLEDGMENTS This work was supported by RFBR under Grant 09-03-00795-a. REFERENCES

(1) Diebold, U. Surf. Sci. Rep. 2003, 48, 53−230. (2) Fujishima, A.; Zhang, X.; Tryk, D. A. Surf. Sci. Rep. 2008, 63, 512−582. (3) Lisachenko, A. A.; Kuznetsov, V. N.; Zakharov, M. N.; Mikhailov, R. V. Kinet. Catal. 2004, 45, 189−199. (4) Lisachenko, A. A.; Mikhailov, R. V. Russ. Tech. Phys. Lett. 2005, 31 (1), 21−24. (5) Lisachenko, A. A.; Mikhailov, R. V.; Basov, L. L.; Shelimov, B. N.; Che, M. J. Phys. Chem. C. 2007, 111, 14440−14447. (6) Mikhaylov, R. V.; Lisachenko, A. A.; Shelimov, B. N.; Kazansky, V. B.; Martra, G.; Alberto, G.; Coluccia, S. J. Phys. Chem. C 2009, 113, 20381−20387. (7) Formenti, M.; Juillet, F.; Meriaudeau, P.; Teichner, S. J. Chem. Tech. 1971, 1, 680−686. (8) A set of interrelated papers: (a) Volodin, A. M.; Cherkashin, A. E.; Zakharenko, V. S. React. Kinet. Catal. Lett. 1979, 11 (2), 103−106. (b) Volodin, A. M.; Cherkashin, A. E.; Zakharenko, V. S. React. Kinet. Catal. Lett. 1979, 11 (3), 221−224. (c) Volodin, A. M.; Cherkashin, A. E.; Zakharenko, V. S. React. Kinet. Catal. Lett. 1979, 11 (3), 277−280. (9) Attwood, A. L.; Murphy, D. M.; Edwards, J. L.; Egerton, T. A.; Harrison, R. W. Res. Chem. Intermed. 2003, 29, 449−465. (10) Berger, T.; Sterrer, M.; Diwald, O.; Knozinger, E. ChemPhysChem 2005, 6, 2104−2112. (11) Henderson, M. A.; Epling, W. S.; Perkins, C. L.; Peden, C. H. F.; Diebold, U. J. Phys. Chem. B 1999, 103, 5328−5337. (12) Lira, E.; Hansen, J. Ø.; Huo, P.; Bechstein, R.; Galliker, P.; Lægsgaard, E.; Hammer, B.; Wendt, S.; Besenbacher, F. Surf. Sci. 2010, 604, 1945−1960. (13) Formenti, M.; Courbon, H.; Juillet, F.; Lissatchenko, A.; Martin, J. R.; Meriaudeau, P.; Teichner, S. J. J. Vac. Sci. Technol. 1972, 9, 947− 952. (14) Yanagisawa, Y.; Ota, Y. Surf. Sci. Lett. 1991, 254, L433−L436.

(15a)

and the observed homoexchange is a result of repeated interactions of gaseous oxygen molecules with long-living Os−. The concentration of exchangeable surface oxygen Os2− can be varied by a reduction/oxidation treatment of the sample (

Investigation of Photostimulated Oxygen Isotope Exchange on TiO2

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