Atomic Hydrogen Activated TiO2 Nanocluster: DFT Calculations - The

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Atomic Hydrogen Activated TiO2 Nanocluster: DFT Calculations Alexey S. Andreev, Vyacheslav N. Kuznetsov, and Yuri V. Chizhov* V.A. Fock Institute of Physics, St. Petersburg State University, Ulyanovskaya Str. 1, St. Petersburg 198504, Russia S Supporting Information *

ABSTRACT: The reduction of TiO2 nanoparticle through interaction with H atom and characterization of resulting defect species with O2 molecule adsorption were investigated by DFT calculations in a molecular cluster approach. The isolated cluster Ti8O16 was used as a nanoparticle model. It was found that interaction between atomic hydrogen and every oxygen atom in Ti8O16 has the following features: (1) formation of stable OH group, (2) low activation energy of the process, and (3) appearance of reduced Ti3+ ion together with one corresponding d-type singly occupied level in the cluster’s “band gap”. Molecular hydrogen, in contrast with its atomic form, weakly interacts only with Ti atoms. Simulation of O2 adsorption on each Ti3+ ion of reduced Ti8O16H clusters shows the following: (1) formation of stable molecular O2− species, (2) the process has no any energetic barrier, and (3) disappearance of Ti3+ defect center and corresponding d-type singly occupied level. The obtained results agree well with general experimental regularities of both H plasma TiO2 reduction and interaction of reduced TiO2 with oxygen. Specifically, calculations of g-tensor provide plausible quantitative description of Ti3+ and O2− paramagnetic species. Consistently the computational approach under consideration can be therefore applied to local surfaces phenomena associated with nanoscaled TiO2.

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INTRODUCTION Properties of point (local) defects on TiO2 surface attract a great attention since they determine the advantages of titania in various technological applications. A few comprehensive reviews dealing with titania intrinsic defect centers have been published by Diebold,1 Diebold and co-workers, 2 and Henderson.3 The main type of TiO2 defect species involves oxygen vacancies at different charged states and defect centers related with Ti3+ ions. Removing surface oxygen atoms forms both the O vacancies and Ti3+ ions, and interaction between H atoms and surface oxygen leads to Ti3+ formation only. However, interaction of hydrogen with TiO2 has been studied to a lesser degree than TiO2 reduction due to O depletion. In this paper the emphasis therefore is on DFT study of TiO2 reduction through interaction with atomic hydrogen. Also, our main concern is the chemical properties of the resulting defect species. It is obvious that quantum chemical study of local defect centers in TiO2 and pathways of their formation is of great interest to researchers. In the present time, such computing techniques as slab calculations,4−9 supercell calculations,10,11 and calculations of free molecular nanoclusters12−19 are commonly used in related investigations. In the past decade, the molecular cluster approach has received much consideration. In particular, Qu and co-workers12−14 have applied high-level DFT calculations to electronic and structural properties of (TiO2)n clusters with n = 1−15 in size dependence. Calatayud and co-authors have analyzed chemical reactivity of TiO2 nanoclusters as a function of size and electronic structure by using H+ and NH3 as probes.15 In recent years, various chemical reactions on TiO2 nanoclusters have © 2012 American Chemical Society

been modeled, namely photoinduced oxidation of water on Ti26O52 cluster,18 hydrolysis reactions on Ti4O8 cluster,19 and dissociation of molecular hydrogen on (TiO2)n clusters (n = 1− 10).17 Of the theoretical methods mentioned above, the molecular cluster approach seemed to be prospective to understanding the nature of local defect centers as well as pathways of their formation in TiO2. We have recently applied the molecular cluster approach to DFT study of surface oxygen vacancies formation and reaction of reduced clusters with NO and CO.20 Specifically, the isolated stoichiometric Ti8O16 cluster was used as a model of TiO2 nanoparticle. This cluster is typical of small model particles considered in the other studies,12,14−16 with it having one of the most stable geometry structures. The total energy of Ti8O16 isomer differs from that of the other isomers14−16 within ∼0.5 eV. Furthermore, differently coordinated Ti and O atoms, being typical of titania surface, are inherent in Ti8O16 cluster (this question is discussed in the Computational Details section ). In addition, small size of the cluster gives a good compromise between low computational cost and possibility to test different types of adsorption sites. In the present work Ti8O16 cluster was used to model TiO2 reduction through interaction with atomic hydrogen and study chemical properties of resulted defect species. Since the applied technique is assumed to be not clear up in every respect, we believe proper experimental data to be very important as a reference base for verification of obtained results. Received: January 31, 2012 Revised: June 15, 2012 Published: August 20, 2012 18139

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Since the 1950s high-temperature treatment in molecular hydrogen has been mainly used for TiO2 reduction.21 Rekoske and Barteau22 and Haerudin and co-authors23 have demonstrated direct experimental evidence for the reduction of powdered TiO2 under H2 heat treatment. Pronounced changes of color from white, in the case of powders, or colorless, in the case of crystals, to blue or dark blue occur under such conditions. Blue color is widely recognized to be a main “fingerprint” of reduced TiO2 specimens. Available in the literature absorption spectra of reduced single crystals and powders have been recently summarized by Kuznetsov and Serpone.24 They showed that blue coloration indicates absorption dominance in the red and near-infrared (NIR) spectral region. Characterization of blue TiO2 by means of electron paramagnetic resonance spectroscopy (EPR) defines color centers responsible for NIR absorption to be associated with various Ti3+ species.25−28 Typically, reduction of TiO2 powders under hydrogen plasma treatment is combined with high-temperature heating (at temperatures about 700 K).29−31 However, Berger and coauthors27 demonstrated a so-called “soft” chemical interaction of atomic hydrogen with surface of TiO2 particles at a lower temperature, T = 77 K. Such treatment resulted in blue coloration and appearance of EPR signals specified to Ti3+ species. Therefore, we consider these experimental results to be the most proper for comparison with data of DFT calculations.

Figure 1. Model of Ti8O16 cluster; atom’s labels runs from 1 to 24; Ti atoms are denoted as gray and O atoms as red; arrows point out coordination number and atom type.

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COMPUTATIONAL DETAILS Calculations were performed with the help of DFT at the B3PW9132−36 level and the GAUSSIAN 03 program.37 Titanium atoms were represented by the LANL2DZ pseudopotential.38 The full-electron 6-31G basis set39 was used to describe O atoms. In order to improve description of atomic and molecular hydrogen adsorption, we applied the 631++G** basis set39 to H atoms, which includes one polarized40 and one diffused41 function. We assume that the applied technique is a reasonable compromise between accuracy and computational efforts. The studied Ti8O16 cluster is found to be rather small, and its linear sizes are 8.87 × 6.68 × 4.71 Å. The cluster structure is amorphous and belongs to C1 symmetry point group. It means that all atoms in the cluster can be considered as chemically nonequivalent. Nevertheless, it is possible to distinguish four types of oxygen atoms, each having specific coordination number (see Figure 1); namely, one four-coordinated atom (O(10)), two three-coordinated ones (O(11) and O(22)), two terminal atoms (O(12) and O(13)), and also 11 bridged atoms (atom’s labels are in parentheses). As for Ti atoms, there are two five-coordinated atoms (Ti(8), Ti(16)) and six fourcoordinated ones. Therefore, the model cluster represents the main types of atom’s coordination which can be found on the real surface of single crystals.1 The ground state of the cluster is singlet. The electronic structure involves occupied and vacant discrete close-lying levels as shown in Figure 2. Highest occupied molecular orbitals (HOMOs) are mainly composed of 2p oxygen states, while lowest unoccupied molecular orbitals (LUMOs) mainly originate from 3d Ti states. The HOMO and LUMO states are separated by an energy gap, which is equal to 4.04 eV. Interactions of H atom and H2 molecule with Ti8O16 cluster as well as oxygen molecule with hydrogenated clusters were simulated in the same way. An adsorbate was placed at definite distance from certain atom of the cluster followed by full

Figure 2. Electronic structure diagram for Ti8O16 cluster.

geometry optimization of the system. In reaction with atomic hydrogen, each atom of Ti8O16 was considered as an adsorption site. The initial interatomic O(N)−H and Ti(N)−H distances were selected to be equal to 1 and 1.5 Å, respectively (here N is the label of cluster’s atoms). Then proper hydrogenated clusters were denoted as Ti8O16H. In reaction with H2, the four-coordinated Ti atoms and seven bridged oxygen atoms were tested as adsorption sites. The initial O(N)−H2 and Ti(N)− H2 distances were equal to those of O(N)−H and Ti(N)−H, respectively. Simulation of O2 interaction with Ti8O16H clusters considered O2 adsorption on reduced titanium atoms. In this case the initial Ti(N)−O2 distance was equal to 1.5 Å; the corresponding clusters were denoted as Ti8O16H−O2. For comparison, interaction of O2 with all unreduced titanium atoms of Ti8O16 cluster was explored as well. When geometry optimization has finished, the adsorption energy of reactant X (H, H2, or O2) on Ti8O16 cluster was defined according to the equation Eads(X/Ti8O16 ) = E(X) + E(Ti8O16 ) − E(Ti8O16 −X)

where E(X) is the isolated adsorbate energy in the ground state, E(Ti8O16) is the total energy of Ti8O16 cluster, and E(Ti8O16− X) is the total energy of the cluster with adsorbed reactant X. The positive value Eads corresponds to exothermic adsorption. The energy of adsorption of O2 on hydrogenated clusters was given by the equation Eads(O2 /Ti8O16 H) = E(O2 ) + E(Ti8O16 H) − E(Ti8O16 H−O2 )

where E(O2) is the energy of neutral isolated molecular oxygen in the ground triplet state, E(Ti8O16H) is the total energy of the hydrogenated Ti8O16H cluster, and E(Ti8O16H−O2) is the total 18140

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energy of the Ti8O16H with adsorbed O2, provided that geometry optimization has been finished. Existence of energy barriers was verified for all reactions considered above. The adsorption activation energies of adsorbate X (H, H2, or O2) were evaluated from full energy profile of the Ti8O16−X system along the adsorption pathway as a function of distance R(O(N)−X) or R(Ti(N)−X). However, geometry of considered systems was not optimized for every R point. We used the following equation to calculate the activation energy: Eact(X/Ti8O16 ) = Emax (Ti8O16 −X) − E(Ti8O16 ) − E(X)

where Emax(Ti8O16−X) is the maximum of total energy for Ti8O16−X system obtained from corresponding energy profile. In the case of molecular oxygen adsorption on hydrogenated clusters the activation energies satisfied the equation

Figure 3. Full energy profile for the most stable hydrogenated Ti8O16H isomer along hydrogen adsorption pathway (R(O(13)−H); Eads is the adsorption energy of H on Ti8O16 cluster (Eads = 3.52 eV), Eact is the adsorption activation energy (Eact = 0.1 eV); E(Ti8O16H) = 0 corresponds to the totally separated configuration.

Eact(O2 /Ti8O16 H) = Emax (Ti8O16 H−O2 ) − E(Ti8O16 H) − E(O2 )

Geometry optimization procedures were carried out under default restrictions for Gaussian 03 program and without any symmetry limits. Harmonic vibrational frequency analysis was performed to find the local energy minimum over the optimized stationary points. The optimized geometry structures of the initial Ti8O16 cluster, 16 hydrogenated Ti8O16H isomers with OH groups, and clusters with adsorbed molecular oxygen Ti8O16H−O2 are described in the Supporting Information. The g-tensors of paramagnetic clusters were calculated by using the GIAO method of the Gaussian program,37 and then obtained tensors were diagonalized.

Electronic structures of the all 16 isomers of Ti8O16H are characterized by the “band gap” equal to ∼4.2 eV (against 4.04 eV “gap” in Ti8O16) and one “middle gap” singly occupied energy level. The electron density plots of these levels and spin density plots of Ti8O16H clusters show that unpaired electrons in the isomers are localized in Ti atoms, which lay close to OH groups as shown in Figure 4. This is accompanied by

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RESULTS Atomic Hydrogen Adsorption on Ti8O16 Cluster. Simulation of atomic hydrogen adsorption on all atoms of Ti8O16 cluster shows that resulting Ti(N)−H bond lengths is found to lay in the range from 2.32 to 2.48 Å with the mean value of 2.42 Å, whereas O(N)−H distance is found to be about 1 Å, which is close to the bond length for OH− radical (0.964 Å).42 Furthermore, formation of OH groups is more energetically preferable than that for TiH species: the energies of H adsorption on oxygen atoms (Eads(H/Ti8O16)) are found to lay in the energy range from 1.37 to 3.52 eV, whereas the energy gain of adsorption on Ti atoms is between 0.03 and 0.09 eV. The adsorption energy also depends on coordination of oxygen atom with lower energy for higher coordination number. Thus, the most stable Ti8O16H isomer has the terminal OH group (O(13)−H), whereas the least stable isomer has the 4coordinated OH (O(10)−H). The activation energy of H adsorption (Eact(H/Ti8O16)) on oxygen atoms is evaluated to be mainly a smaller values near 0.1 eV. Figure 3 illustrates an example of the full energy profile for the most stable isomer of hydrogenated cluster resulted from interaction of H with terminal oxygen atom (O(13)). It is found by calculations that hydrogen adsorption on titanium atoms has zero activation energy. Consequently, the atomic hydrogen strongly reacts with oxygen atoms of stoichiometric cluster and forms OH groups. In turn, Ti−H interaction can be classified as weak physical adsorption. In this case, electronic structure of Ti8O16 cluster is barely perturbed. We will therefore focus only on hydrogenated Ti8O16H clusters with OH groups.

Figure 4. Spin density surface of one Ti8O16H isomer; spin density is shown as mesh.

elongation of corresponding Ti(N)−O(N) bonds by about 0.1 Å (compare with Ti8O16). An appearance of unpaired electron at a Ti atom in each Ti8O16H cluster indicates a reduction of the Ti atom from initial +4 oxidized state into +3 state. Table 1 Table 1. Experimental Data and Computed Values of Main Components of g-Tensors for Ti3+ Sites gxx gyy gzz

experimental data27

calculated valuesa

1.992 1.982 1.960

1.992 ± 0.003 1.979 ± 0.004 1.957 ± 0.008

a

Ti3+ g-tensor values for each Ti8O16H isomer are presented in the Supporting Information.

shows main components of g-tensors for Ti3+ paramagnetic centers. The components are averaged over all Ti8O16H isomers. The right-hand column contains experimental data obtained by Berger and co-authors.27 The energy distance between Ti3+ levels and LUMO states in different isomers ranges from 1.73 to 2.81 eV, with the average being equal to 2.4 eV. Figure 5 shows the electronic structure diagram of the isomers considered above. The difference of the 18141

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tensors’ components of new paramagnetic sites in these clusters correspond to molecular O2− form (see Table 2). Table 2. Experimental Data and Computed Values of Main Components of g-Tensors for O2− Species experimental data gxx gyy gzz

Berger et al.43

Komaguchi et al.28

computed valuesa

2.0033 2.0096 2.0248

2.003 2.010 2.023

2.004 ± 0.002 2.009 ± 0.002 2.05 ± 0.02

O2− g-tensor values for each Ti8O16H−O2 isomer are listed in the Supporting Information.

a

Figure 5. Electronic structure diagram of hydrogenated Ti8O16H clusters with OH groups; dashed lines show the defect states corresponding to Ti3+ ions in different clusters.

It should be noted that the O−O bond length in O2− exceeds that of free O2 molecule by ∼0.1 Å. Such elongation effect is caused by population of antibonding π*-O2 orbital by an extra electron. Molecular Hydrogen Adsorption on Ti8O16 Cluster. Calculations of 11 Ti8O16−H2 clusters (for details see Computational Details section) show that molecular adsorption of H2 on both Ti and O atoms does not require any activation energy. At the same time, molecular hydrogen adsorption on Ti atoms is more energetically preferential than similar process on O atoms. Specifically, the H2 adsorption energy (Eads(H2/ Ti8O16)) on titanium atoms lays in the range from 0.05 to 0.17 eV, whereas the adsorption energy on oxygen atoms is much lower and does not exceed 0.02 eV. The distance between Ti atoms and H2 molecule is about 2.38 Å. The O(N)−H2 length is no less than 2.7 Å and significantly exceeds the O(N)−H bond length in Ti8O16H clusters (∼1 Å). In the case of H2 adsorption, the Ti3+ defect species are not formed, since only a minor perturbation of Ti8O16 electronic structure occurs. Thus, interaction between H2 and stoichiometric Ti8O16 cluster can be characterized as a weak physical adsorption.

“band gaps” of Ti8O16H is found to be less than the difference of the positions of Ti3+ levels. That is why “band gap” of each Ti8O16H is considered as constant equal to the mean value 4.2 eV. The exact positions of Ti3+ levels and HOMO−LUMO gaps for each isomer are listed in the Supporting Information. In other words, there is a set of comparable and distinct Ti3+ electronic defect states. Surprisingly, it was found that differences of main components of computed g-tensors for proper Ti3+ ions are negligible. Moreover, in contrast with the case of Eads(H/Ti8O16), there is no distinct link between energies of Ti3+ defect states and coordination numbers of proper Ti3+ ions or OH groups. Molecular Oxygen Adsorption on H-Activated Ti8O16 Cluster. It is common knowledge that surface or subsurface Ti3+ sites intensively interacts with molecular oxygen. Such an interaction yields the pronounced results, namely disappearance of these sites and appearance of adsorbed O2− species. For details see the Discussion section. Because of this, we were interested to simulate these phenomena with Ti8O16H isomers by means of the proposed molecular cluster approach. As expected, calculations shows that molecular oxygen interacts with reduced Ti3+ sites, and in turn, stable Ti8O16H−O2 clusters are formed. Figure 6 shows that the

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DISCUSSION Interaction of Atomic and Molecular Hydrogen with TiO2 Nanoparticles. Proper experimental data are of great importance for correct validity assessment of the molecular cluster approach for modeling of chemical reactions on surface. To the best of our knowledge, the results of Berger and coauthors27 remain more applicable for comparison with our calculations. The obtained experimental data demonstrate a socalled “soft” chemical interaction between atomic hydrogen and the anatase surface. Nanoparticles of size 13 nm were dehydroxylated in a high vacuum at T = 870 K, oxidized in O2, and then exposed at T = 77 K to atomic hydrogen produced by microwave discharge. Under such conditions, H adsorption occurred spontaneously. Sample’s color changed from white to blue and the narrow feature appeared in the EPR spectrum at g = 1.992. This signal is ascribed to electrons located on 3d orbitals of titanium in anatase. Blue color and paramagnetic properties are both stable at room temperature in vacuum. The proposed scheme27 of hydrogen reaction with dehydroxylated TiO2 includes formation of Ti3+ species and OH groups. It should be noted that neither a significant adsorption nor any paramagnetic species was found experimentally in presence of molecular hydrogen at T = 77 K. Taken together, the results of our calculations agree nicely with the experimental data obtained by Berger and coauthors.27 Spontaneous H adsorption at T = 77 K corresponds

Figure 6. Spin density surface of Ti8O16H−O2 isomer; spin density is indicated by mesh.

adsorbed oxygen does not dissociate and remains in molecular form. The adsorption energy values (Eads(O2/Ti8O16H) run from 0.88 to 1.96 eV. For comparison, the energy gain of O2 adsorption at Ti4+ sites within the initial Ti8O16 cluster is much lower and does not exceed 0.36 eV. In both cases the energy barrier vanishes for O2 adsorption. Calculations for 16 Ti8O16H−O2 clusters yield the following key results: (i) in all isomers, the energy level corresponding to Ti3+ (3d-Ti level) is absent; (ii) in every event, the unpaired electron is localized at adsorbed oxygen molecule; (iii) g18142

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energy of adsorption on 3-coordinated oxygen atoms are 1.76 eV per atom.5 In contrast, the energy of hydrogen adsorption on Ti atoms is considerably lower, 0.01 eV.5 The molecular cluster models often demonstrate presence of terminal oxygen atoms, which show heavy chemical activity. For very large series of (TiO2)n (n = 1−24) clusters the adsorption energy of H atom on terminal oxygen atoms is about 3.2 eV, while bridged oxygen atoms are less active (Eads ≈ 2 eV).16 Note that these values are close to values obtained in the present work. Interaction of Molecular Oxygen with Reduced TiO2. In this section the electronic and chemical effects of interaction with molecular oxygen on the Ti8O16H cluster will be examined, and calculation results will be compared with experimental data. As calculations show, spontaneous (without any energetic barrier) chemisorption of O2 on each Ti3+ site of reduced Ti8O16H clusters is accompanied by two pronounced events: disappearance of Ti3+ defect state in the cluster’s “band gap” and simultaneous appearance of adsorbed O2− species. Note that besides atomic H activation there are other pathways of TiO2 reduction and formation of the Ti3+ sites including heat treatment in vacuum H2 or CO. Of the mentioned phenomena accompanied by O 2 chemisorption, the disappearance of Ti3+ states in the cluster’s “band gap” seems more noticeable because disappearance (or strong decreasing) of a number of surface (or subsurface) Ti3+ defect ions results in a strong decrease of intensity of NIR absorption bands and blue color vanishing as well, i.e., spectralselective bleaching. In such cases the color of reduced samples changes from blue to white or to yellow (if reduced TiO2 sample contains significant number of defects absorbing in the visible region, such as oxygen vacancies). For example, blue coloration under heat treatment of TiO2 P25 in H2 at T > 470 K and immediate color change to white-gray after O2 admission have been observed in the study of Haerudin and co-authors.23 To the best of our knowledge, for the first time results of spectral study on bleaching phenomena have been published in 1996 by Kuznetsov and Krytitskaya.44 Reduction of TiO2 P25 in a H2 or CO at T > 700 K resulted in very broad absorption spectrum (a so-called difference diffuse reflectance spectrum) in the range from ∼3.0 to 0.5 eV with the single maximum in the NIR region at 1.27 eV. Subsequent admission of O2 at 10−2 Torr in a dark caused by extensive oxygen adsorption and bleaching of the sample in the NIR region. As a result, the absorption band at 2.75 eV became dominant in the spectrum.44 Recently, Komaguchi and coauthors28 have reported the difference diffuse reflectance (absorption) spectrum of rutile STR-60C H2 treated at 773 K with typical of the reduced TiO2 maximum in the NIR region at ∼0.97 eV and a shoulder at ∼3.0 eV. Exposure of the reduced sample in O2 at the room temperature produced significant bleaching only in the NIR region. Investigation of the sample, which has been also specified by means of EPR spectroscopy for all treatment stages, presents a major achievement of the work. This favors the more extensive description of reduction−bleaching processes. The EPR spectrum of H2 heat treated rutile TiO2 shows a broad signal at g ≈ 1.96.28 As is clear from the authors’ results and results of other researches mentioned above, the EPR signal is attributed to the Ti 3+ ions (d1 electronic configuration). As for our discussion, an important point is that the Ti3+ EPR signal intensity decreased by a factor 20 in the presence of O2 with simultaneous bleaching of the sample

to the insignificant activation energy in the model of H adsorption. Appearance of Ti3+ species is a prominent result of both experimental and computational data. The calculations of the g-tensor provide an excellent example of plausible quantitative description of the Ti3+ defects species (Table 1). The computed g-values agree well with that obtained by Berger and co-authors for H plasma treated by TiO2 nanoparticles. Moreover, both TiO2 nanoparticles and the Ti8O16 nanocluster show a similar weak reply on adsorption of molecular H2. At the same time, the accordance between blue color of H plasma treated TiO2 samples27 and calculated positions of the Ti3+ states in the cluster’s “band gap” needs to be discussed in detail. It is common knowledge that blue or dark blue color is a main feature of TiO2 specimens reduced under various treatments. Earlier, optical and EPR spectral studies allowed absorption in the NIR region to be assigned to the defect sites related with Ti3+ ions.25,26,28 Analysis of absorption spectra of reduced TiO2 (both anatase and rutile crystals and powders as well) being available in the literature showed that such spectra consist of six strongly overlapped absorption bands, with the spectral maximum (hνmax) lying in the range from ∼2.9 to ∼0.8 eV.24 Dominant absorption at hν < 2 eV (i.e., the bands with hνmax at 1.5−1.7 and ∼1.2 eV) corresponds to blue color (ref 24 and references therein). Note that the position of the Ti3+ states in the cluster’s “band gap” relative to the LUMO state and maxima of the absorption bands related with Ti3+ defect species in the NIR region differ rather noticeably. While the minimum difference is small (1.73 eV (Figure 5) against hνmax = 1.5−1.7 eV), the Ti3+ state’s position taken on average (2.4 ± 0.3 eV) exceeds hνmax by at least 1 eV. This problem is very popular when comparing optical experimental results with computational data. On the one hand, the optical spectra of blue or dark blue TiO2s mentioned above have been obtained for specimens, which differed significantly from the model nanocluster. On the other hand, the problem of correct DFT calculations of both the band gap and defect states in the gap for TiO2 is well-known.4,6,10,11 Solving this problem in the present study was far from our aims. Note that results obtained in the present work are in a qualitative agreement with results of calculations carried out by the other researches. Leconte and co-authors8 have studied the hydrogenation of rutile surface (the hydrogen coverage was θ = 1) using slab model and both HF and DFT methods. They have showed that adsorption of hydrogen atoms only on oxygen atoms was more efficient than the pair adsorption on both oxygen and titanium atoms. Moreover, the first adsorption mode resulted in reduction of some Ti atoms from +4 to +3 formal oxidation state, when the second mode was irreducible. Di Valentin and co-authors6 have showed as well that the electronic structure of hydroxylated TiO2 surface (the hydrogen coverage was θ = 0.5) was characterized by the presence of populated defect bands in the middle of the band gap corresponding to different Ti atoms. The calculations give totally localized solutions, where extra electrons from H atoms are trapped by different Ti atoms reducing them to +3 oxidation state. This trapping process induces distortion of proper Ti−O bonds on ∼0.1 Å. According to slab calculations,5 there are evidence that the bridged surface oxygen atoms are more reactive toward atomic hydrogen than the 3-coordinated ones. Indeed, the adsorption energy of H atom obtained on bridged O atoms for total covered surface are 2.52 eV per one H atom,5 whereas the 18143

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in the NIR region. Moreover, new EPR signal with gxx = 2.003, gyy = 2.010, and gzz = 2.023 concurrently appeared. Authors attribute the species responsible for the EPR signal to adsorbed O2−.28 Thus, we note that calculations under discussion making for g-tensor of Ti8O16H−O2 clusters (Table 2) validate the approach applied to quantitative description of surface species (O2−) resulting from two consequent events, namely, reduction of TiO2 nanoparticle and consequent O2 adsorption. A further handy parameter for comparison of modeled results with experimental data is the computational adsorption energy of O2. It was found that Eads(O2/Ti8O16H) lays in the range from 0.88 to 1.96 eV. These values can be compared with the values of the activation energy of desorption (or simply energy of desorption) Edes of chemisorbed O2. To obtain Edes the mass spectrometric measurements of temperature-programmed desorption (TPD) are generally used.45 The TPD of oxygen from the surface of vacuum-reduced rutile single crystal is thoroughly studied by Henderson and co-workers.46 The TPD spectrum was found to consist of single pick at Tmax = 410 K related to O2− species of desorption energy 1.12 eV. The TPD spectra of oxygen chemisorbed on the surface of reduced powders are more complex. The reason is that they include additional TPD bands, and a band of maximum at 410 K remains dominant in all cases. Maxima of oxygen TPD spectrum lay in the range from ∼410 K44,47 to ∼450 K.23 Estimation of Edes with the help of simple expression Edes ≈ 25kTmax, where k is Boltzmann’s constant,42 gives the values of desorption energy equal to ∼0.9−1.0 eV. Thus, the simulation of formation of molecular adsorbed O2− species on reduced TiO2 clusters is in line with experimental data.

each Ti8O16H−O2 isomer. This material is available free of charge via the Internet at http://pubs.acs.org.

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*Tel 8 (812) 4287500; fax 8 (812) 4287240, e-mail [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Prof. Alexei Emeline and Prof. Vladimir Ryabchuk for fruitful discussions. The present work was supported by Russian Foundation of Basic Research (Grant 10-03-00638-a) and St. Petersburg State University (Grant 11.37.25.2011). Computational resources were provided by Service of Informational Technologies of St. Petersburg State University (St. Petersburg, Russia).

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CONCLUDING REMARKS The reduction of TiO2 nanoparticle through interaction with atomic hydrogen and the characterization of resulting Ti3+ defect species by molecular oxygen adsorption were investigated by DFT calculations in molecular cluster approach. The calculations show that atomic hydrogen reacts with each surface O atom of Ti8O16 and forms stable OH groups without significant activation energy. This is accompanied by reduction of titanium atoms from +4 to +3 oxidation state. In parallel, here is appearance of corresponding d-type singly occupied levels with different energy in the “band gap” of Ti8O16 cluster. Molecular oxygen reacts with the reduced titanium atoms and forms stable molecular O2− species and oxidizes reduced titanium atoms to initial +4 state. Meanwhile, the set of d-type levels in the “band gap” of Ti8O16 cluster disappears. These results are in agreement with the main experimental regularities of both H plasma reduction and interaction of reduced TiO2 with oxygen. Thus, the molecular cluster approach can be reasonably used to describe local surfaces phenomena with nanoscaled TiO2.

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AUTHOR INFORMATION

Corresponding Author

ASSOCIATED CONTENT

S Supporting Information *

Optimized geometry structure of initial Ti8O16 cluster, 16 hydrogenated Ti8O16H isomers with OH groups and clusters with adsorbed molecular oxygen Ti8O16H−O2; activation energies of adsorption and adsorption energies of H atom on oxygen atoms, the clusters’ “band gap” values, the positions of Ti3+ levels in the clusters’ “band gap”, and g-tensor values of Ti3+ ions for each Ti8O16H isomer; adsorption energies of O2 molecule on Ti3+ ions and g-tensor values of O2− species for 18144

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The Journal of Physical Chemistry C

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