Localization of Photoexcited Electrons and Holes on Low Coordinated

Article pubs.acs.org/JPCC

Localization of Photoexcited Electrons and Holes on Low Coordinated Ti and O Sites in Free and Supported TiO2 Nanoclusters Michael Nolan,*,† Anna Iwaszuk,† and Kimberly A. Gray‡ †

Tyndall National Institute, University College Cork, Lee Maltings, Prospect Row, Cork, Ireland Department of Civil and Environmental Engineering, Northwestern University, Evanston, Illinois 60208, United States

‡

ABSTRACT: Photocatalysis is being intensively studied for reactions such as water splitting and CO2 reduction, where absorption of light in a semiconductor such as TiO2 produces electrons and holes that can drive chemical reactions. Efficiencies on bulk materials, however, are too low to be of practical use. In recent years, low dimensional structures such as nanoclusters or nanotubes, displaying metal and oxygen coordination environments very different from the bulk, show promise for improved photocatalytic activities. Key to this is the presence of low coordinated metal and oxygen sites which can act as both charge carrier trapping sites and active sites with target molecules such as water or CO2. This paper presents the results of a density functional theory, with Hubbard U correction (DFT+U), study of electron and hole localization in free and metal oxide-supported TiO2 nanoclusters that display low coordinated titanium and oxygen sites. In free TiO2 nanoclusters, electrons and holes preferentially localize at 4fold coordinated Ti and titanyl, i.e., singly coordinated, oxygen sites in TiO2. For TiO2 nanoclusters supported on rutile (110) electrons preferentially trap on a Ti site in the rutile surface and holes are trapped on the nanocluster−preferentially on titanyl oxygen, if present, and a 2-fold coordinated oxygen otherwise. Our analysis of La2O3 and SiO2 surfaces modified with a Ti5O10 nanocluster shows that electrons and holes are trapped on the TiO2 nanocluster, with electrons residing on a low coordinated Ti site at the interface between TiO2 and the support. Holes are trapped on low coordinated oxygen sites. An important finding is that the modification of the support oxides with these TiO2 nanoclusters induces a band gap narrowing over the unmodified oxide, which will induce a red shift in light absorption. Overall, these studies provide important insights for the design of improved photocatalysts, in that we should seek structures with low coordinated metal and/or oxygen sites that can trap electrons and holes and be useful in adsorbing and activating molecules of interest in photocatalysis.

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INTRODUCTION TiO2 defines the paradigm of photocatalytically active metal oxides1−3 and has been the subject of numerous studies in photocatalytic water splitting,4,5 CO2 reduction,6−9 selfcleaning surfaces,10 and pollutant control.11,12 Mixed phase rutile−anatase has been widely studied in an attempt to understand its enhanced photocatalytic activity over single phase TiO2.13−16 A mixed phase TiO2 composite confers a variety of advantages over single phase materials that includes relative band alignments, which allow the separation of photogenerated electrons and holes as well as the formation of low coordinated interface sites, which are likely catalytic active sites.13−16 Bulk rutile and anatase TiO2 are composed of 6-coordinated Ti and 3-coordinated oxygen species. Cleaving bulk TiO2 to form, for example, the rutile (110) or anatase (101) surfaces produces Ti sites that display reduced coordination at the surface, namely 5-fold and 2-fold coordinated Ti and O sites, respectively. Nanostructures such as nanoclusters and nanotubes display a higher proportion of low coordinated Ti sites that are crucial for reactivity.17−19 Recently, Lee and Kanai20 used first-principles density functional theory (DFT) simulations of anatase (101) and a 1 nm diameter “quantum dot” (QD) cut from anatase to examine the impact of Ti © 2014 American Chemical Society

coordination on the activity for CO2 reduction. The anatase QD displayed 4-fold coordinated Ti sites, while the anatase (101) surface showed only 5-fold coordinated Ti. These authors found that the presence of the 4-fold coordinated Ti sites was critical in providing active sites for CO2 to react, with energy barriers for HCOOH formation significantly reduced over the anatase (101) surface. Tetrahedarally coordinated Ti species are present in small TiO2 nanoclusters that are well studied with DFT.21,22 However, DFT studies tend to focus on how material properties such as energy gaps and cohesive energies change with nanocluster size and converge to the bulk properties. The presence of low coordinated Ti in nanoclusters and its impact on reactivity have been less studied. Posternak et al.23 have studied water adsorption at nanoclusters cut from anatase and highlighted the importance of the low coordinated Ti edge/ corner sites as reactive sites for dissociative water adsorption. In silicates containing Ti,24−26 the presence of 4 coordinated titanium sites is key to the catalytic activity of these materials. Electron paramagnetic resonance (EPR) studies of rutile− Received: September 1, 2014 Revised: November 3, 2014 Published: November 6, 2014 27890

dx.doi.org/10.1021/jp508822v | J. Phys. Chem. C 2014, 118, 27890−27900

The Journal of Physical Chemistry C

Article

and conduction band edges in the nanocluster-modified composite structures, and (3) the presence of low coordinated metal and oxygen sites. These insights into material structure will allow tailoring the design of catalyst structures for activation of molecules and targeted chemical conversion using light.

anatase nanocomposites have identified the presence of electron and hole trapping sites at the phase interface, in particular 4-fold coordinated titanium species and suggested that these are catalytic “hot-spots.”.13−16 Finally, TiO2 nanotubes also have low coordinated Ti sites and are also quite reactive, for example, in CO2 activation.16,27 The presence of low coordinated Ti and O sites is important in photocatalysis for the trapping and transfer of photogenerated charge carriers. Yet, more work has focused on TiO2 band gap modulation28,29 and rather less attention has been paid to examining the formation and localization of photogenerated electrons and holes in TiO2-based structures. Di Valentin and Selloni presented a model of photoexcited anatase TiO2,30 in which a triplet excited state is imposed on the system promoting an electron to the conduction band (CB) and leaving a hole in the valence band (VB). In this detailed study, the authors showed a preference for localization of holes at the surface of anatase (101), where 2-fold coordinated O sites are present, highlighting the role played by low coordinated atomic sites in hole localization. Recent experimental work has shown that it is now possible to modify semiconductor surfaces with well controlled metal oxide nanoclusters that display nonbulk like atomic coordination, in particular low coordinated metal and/or oxygen sites and are more photocatalytically active than unmodified TiO2.31,32 Our DFT studies, in collaboration with experiment,31−35 have shown the potential of structures composed of TiO2 modified with metal oxide nanoclusters for band gap reduction and improved photocatalytic activity. Furthermore, building on the work of Di Valentin and Selloni30 we have demonstrated that photogenerated electrons and holes can localize onto low coordinated atomic sites on TiO2 modified with nanoclusters of SnO,33 PbOx,34 MgO, and Ga2O3.35 Overall, these studies indicate important implications for the design of improved photocatalysts, in that we should seek structures with low coordinated metal and/or oxygen sites that preferentially trap electrons and holes and may have the ability to activate molecules such as water or carbon dioxide18,20 To make further progress in this area, this paper studies in detail models of the photoexcited state of TiO2 nanoclusters in a number of scenarios. Free nanoclusters display low coordinated Ti and O sites not found in bulk materials and we show the localization of electrons and holes at the undercoordinated Ti and O sites in the free nanoclusters. Building on our previous work,36,37 we then examine models of the rutile (110) surface modified with TiO2 nanoclusters to determine how the localization of electrons and holes is altered when supported on a photoactive surface in comparison to the first scenario (free nanoclusters). We study La2O3 (0001) and SiO2 (0001) surfaces modified with a TiO2 nanocluster, as examples of TiO2 nanoclusters supported on a photocatalytically inactive wide band gap support, and compare band gap changes and electron and hole localization with the same TiO2 nanocluster supported on rutile (110). The substrates chosen also allow us to examine the effect of the substrate valence to conduction band energy gaps on the electronic properties and charge localization of composites formed from these substrates and TiO2 nanoclusters and tune the oxidation or reduction chemistry by the choice of substrate. This work shows that the fate of photoexcited electrons and holes in these model structures is determined by (1) the nature of the oxide support, whether wide or narrow band gap, (2) the nature of the valence

2. COMPUTATIONAL METHODS To model TiO2 nanoclusters adsorbed at TiO2, La2O3, and SiO2 surfaces, we use first-principles density functional theory (DFT) simulations. We employ a three-dimensional periodic slab model of the oxide surfaces within the VASP code38 The valence electrons are described by a plane wave basis set and the cutoff for the kinetic energy is 396 eV; having tested TiO2 nanocluster modified surfaces with a 500 eV plane wave cut off energy, we find that there is no significant effect in increasing the plane wave cut off energy. The core−valence interaction is described by PAW potentials,39 and the number of valence electrons is 4 for Ti and Si and 9 for La, with 6 valence electrons for oxygen. Testing of a 12 valence electron PAW potential for Ti shows no change from the results presented for the 4 valence electron potential; see also ref 40. The exchangecorrelation functional is approximated by the Perdew−Wang 9141 functional; this provides consistency with our earlier work in refs,36,37 but some tests on the bare nanoclusters show little effect arising from the exchange-correlation functional. We use the following Monkhorst−Pack k-point sampling grids: rutile (110) surface with a (4 × 2) surface supercell (2 × 2 × 1), (4 × 4), and (8 × 4) rutile (110) surface supercells Γpoint sampling. The La2O3 (3 × 3) surface supercell and SiO2 (2 × 2) surface supercell use (2 × 2 × 1) k-point sampling grids. To describe partially occupied Ti 3d and O 2p states in TiO2, the DFT+U approach is used, with UTi and UO 2p referring to the U values applied to Ti and O, respectively. In line with our previous work33−37 we use UTi = 4.5 eV and UO 2p = 5.5 eV. The need to use DFT+U to describe consistently the occupied d shells in transition metals is well-known42−47 and is important when we consider situations that involve formation of Ti3+ and the value of UTi is widely used in modeling TiO2,36,37,48,49 giving a consistent description of Ti3+ derived states and atomic geometry. For oxygen, we have previously found that a DFT+U description is needed to correctly describe the nature of partially filled oxygen 2p states that result from creating a valence band hole.50,51 The particular value of UO 2p is derived from refs 52, and 53 and is sufficiently large to describe O 2p hole localization. The importance of applying U to the O 2p arises in the present study when we consider the nature of the photoexcited state of our model structures. The surfaces we use as susbtrates for deposition of TiO2 nanoclusters are (1) TiO2 rutile (110), (2) La2O3 (0001), and (3) SiO2 (0001). There is a good deal of experimental work on TiO2−SiO2,11,54,55 whereas we have not found to date any research on modified La2O3. Our calculated bulk rutile lattice constants are a = b = 4.638 Å, c = 2.973 Å.56 The rutile (110) surface is characterized by 2-fold coordinated bridging O atoms that terminate the surface. In the surface layer there are 5-fold coordinated and 6-fold coordinated surface Ti atoms. In rutile (110), (2 × 4), (4 × 4) and (8 × 4) surface supercell expansions are employed to allow isolated nanoclusters of different sizes to be adsorbed. The La2O3 (0001) surface is modeled similarly to our previous work on ALD growth of La2O3,57 with computed lattice constants of a = b = 3.950 Å, c = 27891



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6.154 Å. It is a (3 × 3) surface supercell expansion, with 4 O− La−O trilayers and is characterized as a type II surface in Takser’s notation.58 The surface is terminated with 3 coordinated oxygen atoms in the outermost layer and with 6 coordinated La atoms in the next sublayer. Our calculated bulk SiO2 lattice constants are a = b = 9.826 Å and c = 5.405 Å and a (2 × 2) surface supercell expansion is employed for the αquartz SiO2 (0001) surface, with 6 layers. A 12 Å vacuum gap is used. The convergence criteria for the electronic and ionic relaxations are 0.0001 eV and 0.02 eV/Å. For the consistency in the calculations, we also applied the same supercell and technical parameters for the free TiO2 nanoclusters. In all our calculations, and similar to the literature on TiO2, the surface models are stoichiometric with no defects and no adsorbed hydroxyl species; these are obviously well-known features in metal oxide surfaces but would increase the complexity of our calculations. We have shown in ref.36 that oxygen vacancies can be more easily formed in supported TiO2 nanoclusters compared to the bare rutile (110) surface so that ultimately consideration will need to be given to these defects. The presence of adsorbed water, whether dissociated or adsorbed in a molecular fashion may be important in determining the favorability of binding of the NCs to the rutile surface, potentially blocking surface adsorption sites. Conversely, the presence of O vacancy defects in the support may help to bind the NC more strongly as oxygen from the NC could bind at the vacancy site. However, a detailed analysis of these issues with beyond the scope of this paper. To build the nanocluster-TiO2 models, we first take nanocluster structures that are generated from a Monte Carlo search of low energy structures of a given composition using a charge equilibriation potential to effciently search the space of nanocluster structures; this has been detailed in refs.36,37 The free nanoclusters are relaxed using DFT within VASP. The resulting relaxed nanoclusters are positioned on the TiO2, La2O3 and SiO2 surfaces in different configurations and relaxed. We determine the most stable cluster-surface structures using the adsorption energy, which is computed as Eads = E((TiO2 ) − MOx ) − {E(TiO2 ) + E(MOx )}

oxides to examine two important questions. First, where will the photoexcited electron and hole be localized (in the surface, in the bulk, on the nanocluster). Second, can the localization and energies associated with electron and hole trapping be predicted from a simple DOS analysis. To do this we employ a model in which a triplet electronic state is imposed, as discussed in detail by others30,35,59 Briefly, this model excites an electron to the CB with a corresponding hole in the VB by imposing a tripet electronic state. In determining the trapping energy and the singlet−triplet excitation energy, the following calculations are required: 1. A single point energy of the triplet at the singlet geometry, giving Eunrelaxed. 2. A full ionic relaxation in the triplet electronic configuration, Erelaxed. 3. A dipole correction perpendicular to the surface plane added to the total energies. Within this computational

(1)

Figure 1. Schematic diagram of the relationship between the energies computed in the photoexcited model of TiO2.

where E((TiO2) − MOx) is the total energy of the TiO2 nanocluster supported on the oxide (MOx) surfaces and E(TiO2) and E(MOx) are the total energies of the free TiO2 nanocluster and the unmodified MOx surfaces. A negative adsorption energy indicates that cluster adsorption is stable. Having examined the adsorption energies of our nanoclustermodified surfaces, we select the most stable structures for detailed discussion in the remainder of the paper. The TiO2 NC may take a particular structure in the gas phase but this can, and generally does, change upon adsorption as new bonds are formed between the surface and the TiO2 NC, thus changing the coordination of Ti and O atoms in the NC and also the shape of the NC. For a selection of TiO2 NCs on rutile (110), we tested in refs 36 and 37 if the precise adsorption structure can impact on the energy gap of the composite system. We find that the key point is the formation of interfacial bonds between the NC and the surface with strong adsorption energies and this leads to gap changes that are not so senstitive to the precise adsorption structure, so long as the NC is strongly adsorbed. For TiO2 nanocluster-modified TiO2 some more details on different structures are presented in refs 36 and 37. We study a model of the photoexcited electronic state of the free TiO2 nanocluster and TiO2-nanocluster modified metal

setup, the following energies are calculated and indicated schematically in Figure 1. The Singlet−Triplet Vertical Unrelaxed Excitation Energy. Evertical = Esinglet − Eunrelaxed, where the singlet (the ground state) is fully relaxed and the triplet is fixed at the ground state geometry. This corresponds to the simple VB−CB energy gap from the density of states. The Singlet−Triplet Vertical Excitation Energy. Eexcite = singlet − Erelaxed, where both the singlet and triplet electronic E states are fully relaxed. The change in this energy with respect to the bare surface allows us to determine the effect of the surface modification on the energy gap of TiO2, giving a crude approximation to the excitation energy. The Triplet Relaxation (Carrier Trapping) Energy. Erelax = Erelaxed − Eunrelaxed, where the first energy is the fully relaxed triplet and the second energy is the triplet state at the singlet geometry (from point 1 above), so that Erelax is the energy gained when the electron and hole are trapped at their metal and oxygen sites upon structural relaxation. 27892



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An important consideration is the computed values of these energies. In the present case, a band gap underestimation is present arising using the DFT+U approach, in which the values of U are chosen to localize electrons and holes rather than reproduce the band gap of bulk TiO2. The relaxed singlet− triplet energy difference is smaller than the simple valenceconduction band energy gap, as it includes ionic relaxations and polaron formation in the response to “exciting” the electron. The vertical, unrelaxed singlet−triplet energy difference corresponds to the ground state VB−CB gap and this has also been discussed for rutile (110).54 Thus, the key quantity we focus on is the comparison of the computed energies between different structures. We can state with confidence that a reduction in the singlet−triplet energy difference arising from surface modification of a metal oxide, relative to the unmodified metal oxide, will correspond to light absorption at lower energies in the modified oxide.

Figure 2. Relaxed atomic structure of the TiO2 nanoclusters considered in this work in their singlet ground state: Ti2O4, Ti3O6, Ti4O8, Ti5O10, Ti8O16, Ti16O32, and Ti30O60. In this and subsequent figures, Ti is indicated by a gray sphere and oxygen by a red sphere.

3. RESULTS 3.1. Electron and Hole Localization in Free TiO2 Nanoclusters. For the free subnm diameter TiO2 nanoclusters with compositions Ti2O4, Ti3O6, Ti4O8, Ti5O10, Ti8O16, Ti16O32, and Ti30O60, we have relaxed their atomic structures in the singlet ground state and the lowest energy triplet state using the DFT+U setup described in section 2. The binding energy per TiO2 unit, Eb, in the free nanoclusters has been calculated from Eb = E[(TiO2 )n ] − nE(TiO2 )/n

Table 2 presents the vertical singlet−triplet energy difference, Evertical, the relaxed singlet−triplet energy difference, ES‑T, and Table 2. Vertical Singlet−Triplet Energy Difference (Evertical), the Relaxed Singlet−Triplet Energy Difference (ES‑T), and the Relaxation Energy (Erelax) for Free TiO2 Nanoclustersa

(2)

where E[(TiO2)n] is the energy of the free TiO2 nanocluster, E(TiO2) is the energy of a TiO2 molecule, and n is the number of TiO2 units in the free nanocluster. The resulting binding energies are given in Table 1 and show that the binding energy increases as the size of the TiO2 nanocluster increases, similar to earlier studies in refs21,22 Table 1. Binding Energy of TiO2 Nanoclusters Relative to a TiO2 Trimer in eV Computed using eq 2 E /eV

2 3 4 5 6 8 16 30

−2.5 −3.34 −3.53 −3.99 −3.73 −4.42 −4.90 −5.10

Evertical/eV

ES‑T/eV

Erelax/eV

rutile (110) Ti2O4 Ti3O6 Ti4O8 Ti5O10 Ti8O16 Ti16O32 Ti30O60

2.21 2.10 1.98 1.77 2.11 2.23 1.92 1.67

1.69 0.87 0.58 0.43 0.91 1.29 0.1 0.30

0.52 1.23 1.40 1.34 1.20 0.94 1.82 1.37

a

These quantities are defined in Section 2. Results for unmodified rutile (110) are also shown for comparison.

b

n in (TiO2)n nanocluster

TiO2 nanocluster

the relaxation energy, Erelax, for the free TiO2 nanoclusters. The magnitude of the vertical singlet−triplet energy is similar to that obtained from a simple VB−CB energy difference for each cluster, and we see the well-known dependence of this quantity on the size of the nanocluster. The relaxed singlet−triplet energies show a similar dependence to the vertical energies on the size of the nanocluster size. The relaxation energies are rather large for all nanoclusters indicating a substantial relaxation of the geometry that we will show below is associated with localization of the electron and hole and the response of the local geometry in the nanocluster to the formation of these polarons. For reference, the relaxation energy in the triplet state for the bare rutile (110) surface is much smaller, at 0.52 eV, within the same DFT+U setup as this paper. This difference arises from the nanoclusters having more flexibility to relax in response to the presence of the electron and hole. Figure 2 shows the spin density plots for the free TiO2 nanoclusters in the relaxed triplet state. The spin density plot allows us to determine the location of the electron and hole in the triplet state. In all nanoclusters, the electron localizes onto a single Ti site, which is reduced from a Ti4+ oxidation state to a

The atomic structures of the singlet ground state are shown in Figure 2. We have previously discussed the ground state properties of these nanoclusters in our earlier work36,37 and note here that the key findings are (1) the presence of low coordinated Ti and O sites in the nanoclusters, in particular singly coordinated terminal titanyl oxygen and 3- and 4-fold coordinated Ti, (2) the atomic structure at this length scale, which depends on the size of the nanocluster, (3) the electronic properties such as energy gaps, which also depend on the size of the nanocluster.21,22 These points are important for discussing the effect of nanocluster modification of oxides surfaces and electron and hole localization. 27893



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Ti3+ oxidation state and displays a typical 3d-like spin density isosurface. These Ti sites are either 3-fold or 4-fold coordinated. The computed Bader charges60 on these sites are ca. 1.65 electrons, compared to 1.3 electrons for Ti4+ sites, and the computed spin magnetization is 0.95 electrons on this Ti site, which is consistent with the presence of a reduced Ti3+ species. The geometry around these Ti3+ species shows that the Ti3+−O distances are elongated by ca. 0.1 Å compared to the Ti4+−O distances, which is consistent with the larger ionic radius of the reduced Ti3+ species. These low coordinated Ti sites may be important for the photocatalytic activity of TiO2, acting as sites where an electron can trap and a molecule can adsorb and react. These results show that the size of the TiO2 nanocluster is one approach to modulate the band gap and furthermore the propensity for electrons in nanostructured TiO2 to localize at low coordinated Ti sites, indicating that subnm nanoclusters with low coordinated Ti sites have potential in the design of next generation photocatalysts. Turning now to the oxygen hole, Figure 3 shows that the oxygen hole is strongly localized for all nanoclusters. They key

3.2. Electron and Hole Localization in Rutile (110) Modified with TiO2 Nanoclusters. Having shown that electrons and holes preferentially localize onto low coordinated Ti and O sites in free TiO2 nanoclusters, we now examine electron and hole localization in the rutile (110) surface modified with a representative selection TiO2 nanoclusters. In Figure 4, we show the atomic structure for rutile (110)

Figure 4. Atomic structure of rutile (110) modified with the Ti2O4, Ti3O6, Ti4O8, and Ti5O10 nanoclusters. The color coding is the same as Figure 2

modified with the Ti2O4, Ti3O6, Ti4O8, and Ti5O10 nanoclusters, as structures that capture the essential properties of modified TiO2 and that are also computationally feasible. In our previous work in refs 31 and 32, we showed that 1. Adsorption of the TiO2 nanoclusters at rutile (110) maintains some of the low coordinated Ti and oxygen sites present in the free nanoclusters. The most prominent of these low coordinated sites are 4-fold coordinated Ti and 2-fold coordinated and terminal oxygen, all in the nanocluster. 2. The large adsorption energies for TiO2 nanocluster adsorption at rutile (110) (between −2.72 and −7 eV36,37) are driven by formation of new nanocluster to surface bonds.36,37 3. The projected electronic density of states (PEDOS) for the Ti 3d and O 2p states in the TiO2 nanocluster and the rutile (110) surface in Figure 5a−d shows that there are new states present above the valence band edge of the rutile (110) surface which arise from the modification of the surface with the nanoclusters. This has the effect of broadening the valence band (VB), pushing the VB edge closer to the CB and reducing the band gap, with the potential to induce visible light absorption. The formation of the new electronic states that induce band gap narrowing over bare TiO2 arises from the strong interaction between the nanocluster and TiO2, in particular the formation of interfacial Ti−O bonds and are not as a result of sensitization of TiO2 with the nanoclusters, which would be similar to quantum dots, but are unique to nanocluster modified TiO232,62 4. Finally, the extent of band gap narrowing is dependent on the size of the adsorbed TiO2 nanocluster. Yet, there is no apparent obvious trend with the size of the nanocluster, which is typical for these subnm sized TiO2 nanoclusters.21,22

Figure 3. Spin density plots for the free TiO2 nanoclusters in the excited state. The spin density isosurfaces are orange and enclose spin densities of up to 0.2 eV/Å3.

finding is that the site for oxygen localization is a singly coordinated terminal oxygen; the exceptions to this are the Ti4O8 and Ti30O60 nanoclusters. In the former, the oxygen hole is spread over two 2-fold coordinated oxygen atoms, while in the latter, the hole is spread over a terminal oxygen atom (80%) and the nearest 2-fold coordinated oxygen atom (20%). For Ti4O8, this hole distribution was also found using time dependent DFT calculations.61 In any case, it is clear that the oxygen hole is strongly localized in free TiO2 nanoclusters. The computed Bader charge on the hole carrying oxygen atoms is 6.8 electrons, compared to 7.3 electrons for a lattice oxygen. The computed spin magnetization on this oxygen is ca. 0.85 electrons (with the exceptions above), with a spin magnetization of less than 0.05 electrons on other oxygen atoms. These charges and spin magnetizations are typical of an oxygen hole polaron. The Ti4+−O− distances in the nanoclusters are elongated by 0.1−0.12 Å over the Ti4+−O2− distances in the ground state, which are consistent with formation of a localized oxygen hole polaron, which displays an elongation in bond distances. 27894



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and the relaxation energy for the nanocluster-modified rutile (110) structures, with a comparison to unmodified rutile (110). If we first compare the vertical energy difference with the simple VB−CB energy difference from the DOS analysis,36,37 this energy shows the same trend, in that all nanocluster modified structures show a reduction in the vertical energy compared to unmodified rutile (110) and this reduction is dependent on the size of the TiO2 nanocluster. The Ti3O6modified surface shows the largest shift of the energy gap toward the visible region. The relaxed singlet−triplet energies are also strongly reduced compared to the unmodified rutile (110) surface, demonstrating that modification with TiO2 nanoclusters will induce a red shift in the wavelength of light absorbed. While a hybrid DFT level calculation of these energies would give a more precise indication of the change in the singlet−triplet energy, such calculations are presently not possible within a plane wave basis for these systems. The relaxation energies in Table 3 reflect the energy gain when the electron and hole localize (see below) and the relaxation of the structure in response to charge localization. The relaxation energies are all significantly larger than the bare rutile (110) surface, which arises from the ability of the nanocluster to undergo stronger relaxations around a localized charge compared to the surface. Again, for the length scales of the TiO2 nanoclusters considered the magnitude of the relaxation energy shows no trends with the size of the TiO2 nanocluster. To examine the location and localization of the photoexcited electron and hole, we present the excess spin density plots in the TiO2 nanocluster modified rutile (110) structures in Figure 6. The spin density plots show that the electron and hole

Figure 5. PEDOS plots for the TiO2 nanocluster modified rutile (110) structures: (a) Ti2O4−TiO2, (b) Ti3O6−TiO2, (c) Ti4O8−TiO2, and (d) Ti5O10-TiO2. The left panel in each case shows the Ti 3d PEDOS decomposed into surface and nanocluster contributions, while the right panel in each case shows the O 2p PEDOS decomposed into surface and nanocluster contributions. The nanocluster PEDOS are multiplied by the factors (either ×10 or ×20, that is 10 times or 20 times) shown in the figure to account for the difference in the number of atoms in the nanocluster compared to the surface. The zero of energy in each case is the Fermi level.

The composition of the valence and conduction band edges leads us to propose that upon excitation electrons will localize on the rutile surface and holes on oxygen of the nanocluster.36,37 Furthermore, from the analysis of the free nanoclusters, if a hole or electron localizes onto the nanocluster, the most likely sites are titanium or oxygen with the lowest coordination, which are 3- or 4-fold Ti and the terminal titanyl oxygen. First we examine the energetics and Table 3 presents the vertical singlet−triplet energy, the relaxed singlet−triplet energy Figure 6. Spin density plots for the photoexcited electron and hole in TiO2 nanocluster modified rutile (110) structures, where the nanocluster composition is indicated in each case. The color coding and spin density isosurface parameters are the same as Figure 3

Table 3. Vertical Singlet−Triplet Energy Difference (Evertical), the Relaxed Singlet−Triplet Energy Difference (ES‑T), and the Relaxation Energy (Erelax) for NanoclusterModified Rutile (110) Structures and Unmodified Rutile (110) as Reference modified TiO2 structure

Evertical/eV

ES‑T/eV

Erelax/eV

unmodified rutile (110) Ti2O4-modified rutile (110) Ti3O6-modified rutile (110) Ti4O8-modified rutile (110) Ti5O10-modified rutile (110)

2.21 2.11 1.61 1.95 2.09

1.69 1.18 0.25 0.71 0.59

0.52 0.93 1.37 1.24 1.50

localize onto a Ti site in the rutile (110) surface and an oxygen site in the nanocluster, respectively. This pattern of electron and hole localization is found for all nanocluster modified structures we have studied. We confirm the proposition from the simple DOS analysis that the electron and hole will localize onto the rutile (110) surface and the TiO2 nanocluster, respectively. The resulting physical separation of the electron and hole will be beneficial for the photocatalytic activity, as it 27895



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4. TIO2 NANOCLUSTER MODIFICATION OF LA2O3 AND SIO2 SUBSTRATES In this section we investigate TiO2 nanocluster modification of insulating, generally nonphotocatalytically active oxides with a wider band gap and thus different valence and conduction band alignments compared to TiO2. We use La2O3 and SiO2 as examples of such substrates. The (0001) surface of each oxide is modified with a Ti5O10 nanocluster, which is a representative subnm TiO2 nanocluster, and we investigate the localization of the photoexcited electron and hole, the associated energies and the change in the VB−CB gap of upon modification in these modified structures, comparing with modified rutile (110). 4.1. TiO2 Nanocluster Modification of La2O3 (0001). Figure 7a shows the atomic structure of the La2O3 (0001)

will inhibit the charge recombination process, in particular in comparison with unmodified rutile TiO2. Experimental results suggest that the photocatalytic activity of TiO2 modified with metal oxide nanoclusters is enhanced over unmodified TiO263,64 and the results of this work provide an origin of these findings. The electron localizes onto a reduced Ti3+ site in the subsurface layer of rutile (110), with a computed Bader charge of 1.8 electrons, compared to 1.24 in the unmodified rutile (110) surface. The computed spin magnetization on the Ti site is 0.95 electrons and less than 0.01 for all other Ti sites. Ti2O4modified rutile (110) shows a different site for localization of the electron; in this case the electron localizes onto a 5-fold coordinate Ti site in the surface. It is well-known from DFTbased studies of Ti3+ formation in rutile (110)65,66 that localization of an electron onto subsurface Ti sites is most favorable, although localization onto a surface Ti site is also possible67 with a small barrier to electron hopping between Ti sites in different layers of rutile (110); this is ca. 0.1 eV.68 Thus, it is quite possible that an electron may occupy surface or subsurface Ti sites. We also found in ref35 that upon oxygen vacancy formation in Ti2O4-modified rutile (110) one electron localizes onto a surface 5-fold coordinate Ti atom. It is possible that the ability of the TiO2 surface to distort the atomic structure and localize an electron on a surface Ti atom is driven by the small size of the supported TiO2 nanocluster, which is not possible in the cases with the larger supported TiO2 nanoclusters. In any case, the key point is the localization of the electron on the TiO2 surface. The Ti−O distances around the Ti3+ site are elongated by 0.1 Å over the Ti4+−O distances, which is again consistent with formation of a localized, reduced Ti3+ species. In all nanocluster-modified structures, the oxygen hole always localizes onto a low coordinated oxygen in the TiO 2 nanocluster. In the case of the supported Ti2O4 nanocluster the hole localizes onto a 2-fold coordinated oxygen in the nanocluster since no terminal oxygen are available in this particular structure. In the remaining TiO2 nanoclusters, the hole localizes onto one of the singly coordinated terminal titanyl oxygen atoms in the nanocluster. This pattern of oxygen hole localization onto oxygen with the lowest possible coordination is the same as described in the free nanoclusters. When supported on rutile (110), the Ti4O8 nanocluster now displays a single localized oxygen hole species compared to having the hole spread over two oxygen atoms in the free nanocluster. The computed Bader charge for the oxygen that carries the localized hole is in the range of 6.65−6.70 electrons, depending on nanocluster size and the computed spin magnetization is ca. 0.80 electrons; the Bader charge and spin magnetization thus confirm the formation of a localization oxygen hole, being typical for this species. Furthermore, the Ti4+−O− distances around the oxygen hole site are elongated by up to 0.2 Å over the Ti4+−O2− distances, which is consistent with formation of a localized, oxygen hole species. In terms of the relaxation energies associated with the electron and hole localization in these structures, the localization of an electron or hole forms a polaron species, which is accompanied by a local distortion of the atomic structure around the hole or electron site. This relaxation accommodates the localized reduced Ti3+ cation or the oxygen hole. The nanoclusters are able to distort more strongly than the free rutile (110) surface and therefore undergo stronger relaxations, giving a notably larger relaxation energy.

Figure 7. (a) Atomic structure and PEDOS of La2O3 (0001) modified with a Ti5O10 nanocluster. (b) PEDOS of Ti5O10-modified La2O3 in which the Ti 3d and O 2p states of TiO2 are shown along with the total La2O3 DOS; the Fermi level set to 0 eV. The color coding is the same as Figure 2, with the La atoms of the La2O3 (001) surface indicated by blue spheres. Titanyl oxygens are labeled O1c, and 4coordinated titanium are labeled Ti4c in part a.

surface modified with a Ti5O10 nanocluster. The adsorption energy of the Ti5O10 nanocluster is −5.45 eV, indicating a strong nanocluster-surface interaction. The adsorption of the Ti5O10 nanocluster induces some distortions to the La2O3 surface as a result of the formation of new interfacial bonds between the TiO2 nanocluster and the surface. For example, the outermost oxygen atoms in La2O3 that form new bonds to Ti atoms in the nanocluster are displaced out of the surface layer, with Ti−O distances of 1.90 and 1.92 Å (typical of Ti−O distances in TiO2) and these surface oxygens have surface La− O distances of 2.40−2.68 Å, which show a notable elongation over the corresponding La−O distance of 2.37 Å in the bare surface. The O−La distances involving oxygen from the TiO2 nanocluster are 2.42 and 2.49 Å, which are again elongated compared to the La−O distances in the bare surface. Examining the atomic structure in the adsorbed nanocluster, there are two terminal oxygen atoms indicated in Figure 7, with Ti−O distances of 1.69 Å, which from the discussion in Sec. 3 could be potential hole localization sites. There are also 4-coordinated Ti species present in the nanocluster, one of which is present in the interface between the La2O3 surface and the Ti5O10 nanocluster, as indicated in Figure 7. These are potential sites for electron localization. The other Ti−O distances in the nanocluster range from 1.89−2.08 Å. The electronic DOS in Figure 7b indicates that the modification of La2O3 with Ti5O10 has a strong effect on the position of the La2O3 conduction band edge, which is different to the effect on modified rutile (110), where no change to the CB was seen. TiO2-derived states are present below the original CB edge of La2O3 which pushes the CB edge of the composite 27896



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low coordinated Ti and oxygen sites. The low coordination of these sites drives the charge carrier localization. It appears also that the preferred site for electron localization is in the interfacial region on a Ti binding to oxygen from both the nanocluster and the La2O3 support. This highlights a difference between the TiO2 and La2O3 supports where the electron localized onto a surface Ti atom in the former case. Thus, electron localization in TiO2 nanocluster modified metal oxides can be controlled by the choice of oxide substrate. The computed energies are shown in Table 4; the DFT energies in this table are taken from relaxed ground state singlet

structure lower in energy. This will have the effect of narrowing the band gap over unmodified La2O3, with a predicted decrease of 2.2 eV. Clearly, however, the band gap underestimation inherent to DFT will play a role in the precise positions of the energy bands, although we have generally found for surface modified TiO2 the DFT band gap reductions are consistent with experiment.31,32 In contrast to modifying TiO2 surfaces with oxide nanoclusters36,37 the PEDOS for La2O3 modified with TiO2 nanoclusters indicates that the VB edge remains dominated by the occupied La2O3 states, although the occupied TiO2 states lie only 0.3 eV lower than the La2O3 surface derived VB edge. We anticipate that upon photoexcitation, electrons will localize onto Ti sites in the TiO2 nanocluster, and it is likely that holes will also localize onto low coordinated oxygen in the TiO2 nanocluster, since these oxygens make up the highest energy O 2p states in Ti5O10 and lie close to the La2O3 VB edge. Such oxygen would also be expected to be better at trapping holes than surface oxygen in the La2O3 surface. Thus, charge localization in these structures may be different to modified rutile TiO2. Figure 8 shows the spin density plot for the relaxed triplet photoexcited state of Ti5O10-modified La2O3 (0001). The

Table 4. Vertical Singlet−Triplet Energy Difference (Evertical), the Relaxed Singlet−Triplet Energy Difference (ES‑T), and the Relaxation Energy (Erelax) for Ti5O10Modified La2O3 (0001) and Unmodified La2O3 (0001) as Reference modified La2O3 Structure

Evertical/eV

ES‑T/eV

Erelax/eV

unmodified La2O3 Ti5O10-modified La2O3

4.4 2.28

3.7 1.75

0.70 0.53

and lowest energy triplet energy differences, and will be smaller than simple Kohn−Sham energy eigenvalue differences determined from a DOS plot. The Ti5O10 nanocluster modification of La2O3 reduces the vertical energy gap by 2.12 eV and reduces then singlet−triplet energy by 1.95 eV. We suggest that a substantial red shift of the wavelength of light absorption will be found for La2O3 modified with TiO2, which is similar in the behavior of rutile TiO2 modified with TiO2 nanoclusters. 4.2. TiO2 Nanocluster Modification of SiO2 (0001). Figure 9a shows the atomic structure of a model SiO2 (0001)

Figure 8. Spin density isosurface for Ti5O10-modified La2O3 (0001) surface, with the Ti3+ and oxygen hole sites indicated. The spin density isosurfaces enclose spin densities up to 0.02 eV/Å.

electron and hole formed after excitation and relaxation are localized onto the TiO2 nanocluster. From the DOS analysis, one would expect that the hole would localize onto a surface oxygen atom in La2O3. For an unrelaxed triplet state we see that the hole is indeed predominantly localized onto surface oxygen atoms in La2O3. However, upon relaxation, the hole now localizes onto an oxygen atom in the nanocluster that was originally 2-fold coordinated, but due to relaxation around the hole, it is now singly coordinated. The origin of this effect is due to the ability of the TiO2 nanocluster to trap holes accompanied by the structural relaxations in the nanocluster upon hole localization. This produces a lower energy minimum than the case where the hole localizes onto La2O3. The Ti4+−O distance is 2.02 Å, typical of a Ti−O distance involving a localized oxygen hole. The photogenerated electron localizes onto a Ti atom in the Ti5O10 nanocluster that was originally identified as a 4-fold coordinated Ti (Figure 7a), and after electron localization this Ti3+ is now 3-fold coordinated with Ti−O distances of 1.93, 1.99, and 2.07 Å around this site that are typical of Ti3+−O distances. The oxygen hole site carries a spin magnetization of 0.80 electrons, while the Ti ion onto which the electron localizes has a spin magnetization of 0.95 electrons. Thus, we can identify a localized oxygen hole and a reduced Ti3+ site in the TiO2 nanocluster, that are present on

Figure 9. (a) Atomic structure and PEDOS of SiO2 (0001) modified with a Ti5O10 nanocluster. (b) PEDOS of Ti5O10-modified SiO2 in which the Ti 3d and O 2p states of TiO2 are shown along with the total SiO2 PEDOS; the Fermi level set to 0 eV. The color coding is the same as Figure 1 with the Si atoms of the SiO2 (0001) surface indicated by yellow spheres. Titanyl oxygen are indicated by O1c and 4fold coordinated titanium by Ti4c.

surface modified with a Ti5O10 nanocluster. The adsorption energy of the Ti5O10 nanocluster is −8.61 eV, indicating a substantial interaction between the nanocluster and the SiO2 surface; a similarly large adsorption energy of −9.86 eV is calculated for the same Ti5O10 nanocluster adsorbed at a larger (4 × 4) surface supercell of the SiO2 (0001) surface. The interaction between the nanocluster and the SiO2 surface also induces a substantial distortion to the SiO2 surface upon formation of the interfacial bonds between the TiO2 nanocluster and the surface. The Ti−O bond lengths to surface oxygen are 1.92, 1.99, 2.03, 2.04, and 2.06 Å. The Si−O bond 27897



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lengths to nanocluster oxygen are 1.56, 1.57, and 1.63 Å. A terminal oxygen is present in the adsorbed Ti5O10 nanocluster, with a Ti−O distance of 1.69 Å. Furthermore, and similar to TiO2-modified La2O3, there is a 4-coordinated Ti species, indicated in Figure 9, that binds to oxygen in the nanocluster and the SiO2 surface. This Ti atom has Ti−O distances of 1.81, 1.92, and 2.04 Å. The electronic DOS in Figure 9b indicates that both the valence and conduction band edges of SiO2 are modified by the presence of the TiO2 nanocluster. The presence of TiO2derived occupied states pushes the valence band edge up in energy by 0.8 eV. The bottom of the conduction band is dominated by the empty Ti 3d states, which lie 0.5 eV lower in energy than the empty SiO2 states. Thus, the character of both the VB and CB edges of SiO2 are modified by surface modification with the TiO2 nanocluster. The energies in Table 5 indicate that modifying SiO2 with a TiO2 nanocluster can

Figure 10. Spin density isosurfaces of Ti5O10−SiO2 after relaxation of the photoexcited state. The spin density isosurfaces enclose spin densities up to 0.02 eV/Å.

Table 5. Vertical Singlet−Triplet Energy Difference (Evertical), the Relaxed Singlet−Triplet Energy Difference (ES‑T), and the Relaxation Energy (Erelax) for NanoclusterModified SiO2 (0001) and Unmodified SiO2 (0001) as Reference modified SiO2 structure

Evertical/eV

ES‑T/eV

Erelax/eV

unmodified SiO2 Ti5O10-modified SiO2

4.7 2.28

3.64 0.71

1.06 1.56

localize onto this interfacial Ti site. It is possible that the formation of electron trapping interfacial 4-fold coordinated Ti sites is a general phenomenon that may be useful for developing future photocatalysts based on modifying metal oxides with TiO2 nanoclusters (as long as TiO2 is not used as the support).

4. CONCLUSIONS There is great interest in low dimensional nanostructured metal oxides for applications in photocatalysis such as water splitting or CO2 reduction. Low dimensional structures such as nanoclusters display metal and oxygen coordination environments very different to the bulk which can be exploited for improved performance in photocatalysis. We presented in this paper the results of DFT+U simulations of free and supported TiO2 nanoclusters that display low coordinated titanium and oxygen sites in both the gas phase and supported on different metal oxide substrates. For both free and supported nanoclusters, a model of the photoexcited state shows that electrons and hole preferentially localize at 4-fold coordinated Ti and singly coordinated, titanyl, oxygen sites. Analysis of TiO2 nanoclusters supported on rutile (110) shows that the surface is a stronger electron trapper than the nanoclusters, with electrons preferentially trapping on a surface or subsurface Ti site, in a similar fashion to the well-known oxygen vacancy in rutile (110). The hole traps onto the titanyl oxygen, if present, and a 2-fold coordinated oxygen otherwise. An examination of La2O3 and SiO2 surfaces modified with a Ti5O10 nanocluster shows that the electron and hole trap on the TiO2 nanocluster, with electrons preferring to reside on a low coordinated interfacial Ti site and holes on titanyl oxygen. The ability of low coordinated metal and oxygen sites to trap electrons and holes, when coupled with suitable band edge positions for reduction or oxidation in the photocatalyst, can be a crucial ingredient for improving the efficiency of photocatalytic CO2 reduction or water oxidation.

reduce the energy gap of SiO2 significantly and give a red shift in the wavelength of light absorbed. For TiO2-modified SiO2, we anticipate that upon photoexcitation, electrons and holes will localize onto the TiO2 nanocluster, with the low coordinated Ti and O sites being the most likely sites for localization. We see how the effect of TiO2 nanocluster modification on the valence and conduction band is determined by the choice of the support. This could arise from the energy band alignments of the TiO2 nanocluster and the substrate oxide. However, band alignments of noninteracting TiO2 nanocluster and oxide support will be modified by the formation of interfacial bonds and subsequent relaxation. We also note that the top of the VB tends to derive from O 2p states localized on the titanyl oxygen atoms, which lie at higher energy than O 2p states derived from oxygen with higher coordination. Thus, the TiO2 nanocluster modification tends to drive the VB to higher energy. The CB edges of La2O3 and SiO2 lie at less negative energies (on an absolute energy scale) compared to TiO2, and therefore, the CB will be derived from TiO2. Figure 10 shows the excess spin density in Ti5O10−SiO2 after relaxation of the photoexcited state. The localization of the electron and hole onto a Ti and O site in the TiO2 nanocluster is clearly visible and is consistent with the simple analysis from the PEDOS. The hole localizes onto the terminal oxygen present in the nanocluster, with a Ti−O− distance of 1.90 Å. For this oxygen, the computed spin magnetization is 0.80 electron, which is typical of an oxygen hole site. The photoexcited electron localizes onto a 4-coordinated Ti site that is present in the interfacial region between the surface and the nanocluster. This Ti site has Ti−O distances of 1.96, 1.98, 2.08, and 2.14 Å and a computed spin magnetization of 0.95 electrons, both consistent with a reduced Ti3+ site. While in this TiO2 nanocluster, there are other 4-coordinated Ti sites (Figure 9a), we find that the photoexcited electron prefers to

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Corresponding Author

*(M.N.) E-mail: [email protected]. Notes

The authors declare no competing financial interest. 27898



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ACKNOWLEDGMENTS We acknowledge support from Science Foundation Ireland (SFI) through the Starting Investigator Research Grant Program, Project “EMOIN”, Grant Number SFI 09/SIRG/ I1620, SFI through the US−Ireland R & D Partnership Program, Grant Number SFI 14/US/E2915, and the European Commission through the COST Action CM1104 “Reducible Metal Oxides, Structure and Function”. We acknowledge access to computing resources at Tyndall provided by SFI and by the SFI and Higher Education Authority funded Irish Centre for High End Computing and the European Commission Partnership in Advanced Computing (PRACE, Contracts RI-261557, RI-283493, and RI-312763) for access to the UYBHM Computer at Istanbul Teknik Universitesi and the JUROPA Computer at Forschungszentrum Juelich through the DECI initiative.

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