Article pubs.acs.org/JPCC
Titania−Silica Interfaces N. Seriani,*,† C. Pinilla,† S. Cereda,‡ A. De Vita,‡ and S. Scandolo§ †
The Abdus Salam International Centre for Theoretical Physics, Strada Costiera 11, 34151 Trieste, Italy Physics Department, King’s College, London Strand, London WC2R2LS, United Kingdom § CNR-IOM Democritos National Simulation Center, Trieste I-34136, Italy ‡
S Supporting Information *
ABSTRACT: The photocatalytic activity of titanium dioxide is sometimes observed to increase upon addition of silica, but the origin of this enhancement is not clear. To shed light on this effect, we investigate the titania/ silica system in case of perfect segregation using density functional theory. Two situations have been considered: a single silica monolayer covering a titania surface and a bulk titania/silica interface. In both cases, we find that the presence of silica strongly modifies the electronic structure of the catalyst. In the case of the bulk interface, the analysis of the projected density of states reveals that interface electronic states give a large contribution to the edges of valence and conduction bands. For a silica monolayer on a (101) surface of anatase, the spatial localization of the states near the edge of the valence band depends on the hydroxylation state of the monolayer. Similar to the bulk case, the hydrogen-free monolayer hosts interface states, whereas in the fully hydroxylated system, these disappear and all top valence band states are located inside the titania slab. These results can be rationalized in terms of the number of Ti atoms available for bonding with bridging oxygens at the interface, suggesting that the ratio of Ti−O−Si and 2Ti−O−Si linkages allowed by the interface bonding topology may play an important role in photoabsorption processes.
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INTRODUCTION Titania is often employed as a photocatalyst.1−10 Its photocatalytic applications range from self-cleaning glasses11 and water-splitting catalysts for hydrogen production12−14 to disinfection15 and water purification.16,17 The underlying mechanism is the same in all these applications: electromagnetic radiation in the ultraviolet spectrum excites electron− hole pairs in the titania, and the resulting charge carriers contribute to the oxidation or degradation of molecules adsorbed on the catalyst surface.18 Addition of silica may increase the photocatalytic activity, but only if the two oxides are in tight contact with each other and not just coexistent after simple mechanical mixing.1 Titania− silica materials have been produced with several morphologies: atomically mixed oxides, zeolites, nanoscopic titania in a silica matrix and (multi)layered thin films.1,19,20 Even in the restricted case of layered thin films, contrasting results are found in the literature; for instance, Miyashita et al.21 observed an improved photocatalytic activity, and others have observed silica to suppress it.22,23 A full rationalization of these different outcomes is still missing. In some cases silica has been suggested to improve the adsorption of the reacting molecules while playing a negligible role in the photoexcitation.1 This hypothesis stems also from the fact that the tendency of mixing is small. Although thin films with TiSiO4 stoichiometry have been obtained,24,25 at temperatures above 600 °C phase separation and anatase crystallization have been observed within 1 h.24 We can therefore conclude that, in a first approximation, any possible effect of silica on photoexcitation must be linked with the presence of a titania/silica interface. © 2012 American Chemical Society
To clarify the effect of silica on the atomic and electronic structure of the catalyst, we have investigated titania/silica phases by means of density functional theory in the case of complete phase segregation and formation of Ti−O−Si bonds at the atomically sharp interface. Our results suggest that the presence of silica strongly modifies the electronic structure of the catalyst and has an influence on the edges of valence and conduction bands. In the case of a bulk interface between pure TiO2 and SiO2, the interface gives a large contribution to the electronic density of states (DOS) near the valence and conduction band edges. This could have consequences for the photoabsorption properties of the material. In the next section we present our computational methods; we next present our results for the bulk titania/silica interface and for a silica monolayer at a titania surface, followed by a conclusive summary.
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COMPUTATIONAL METHODS The ab-initio calculations have been performed within density functional theory (DFT), as implemented in the QuantumEspresso package.26,27 Electronic exchange and correlation have been modeled within the Perdew−Zunger version of the local density approximation (LDA)28 for the bulk systems and in the version of the generalized-gradient approximation (GGA) proposed by Perdew, Burke, and Ernzerhof (PBE)29 for the study of the monolayer at a titania surface. This choice is motivated by the expectation that LDA describes better the Received: February 17, 2012 Revised: April 25, 2012 Published: May 1, 2012 11062
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bulk phases, whereas GGA works better for surfaces. For all elements, Vanderbilt ultrasoft pseudopotentials30 have been employed. An energy cutoff of 60 Ry has been used for the wave functions and 720 Ry for the charge density. For the purpose of this work a cutoff of 40 Ry for the wave functions would have been sufficient, but the choice of a higher cutoff was dictated by our plans to develop a classical potential for these systems on the basis of a force-matching procedure. The latter requires a very high degree of convergence of forces and stresses. A mesh of (8 × 8 × 4) mesh of k-points has been used for integration in the first Brillouin zone cell of anatase TiO2. Optimizations have been performed until forces were smaller than 10−3 au. With DFT-LDA, lattice constants of 4.54 and 2.92 Å (exp 4.59 and 2.96 Å31) have been found for bulk TiO2 rutile upon structural optimization, in agreement with previous calculations.32 Lattice constants of 3.74 and 9.45 Å have been obtained for bulk anatase (exp 3.78 and 9.50 Å31) and of 4.85 and 5.37 Å for α-quartz (exp 4.92 and 5.41 Å33), again in agreement with previous calculations (refs 34, 35 and ref 36, respectively).
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RESULTS AND DISCUSSION Bulk Interface. We constructed our model of a sharp interface between anatase and amorphous silica so that, away from the interface, the perfect atomistic structure of the two compounds is preserved. The proposed model, shown in Figure 1, was built in an attempt to minimize the number of undercoordinated Ti atoms and has two key features which should be expected from a realistic titania/silica system: first, it is built from an anatase−TiO2(101) surface, which is the most stable surface35 of the most active titania phase;18 second, each Si atom is coordinated to 4 O atoms within a tetrahedral unit. After full relaxation of the repeated cell geometry and atomic positions, Ti−Ti and O−O distances in the anatase slab system differ from their bulk values by less than 4%. The resulting average Ti−O distance is 1.92 Å to be compared with the value of 1.94 Å we find in bulk anatase. Since the strain state of the interface may depend on macroscopic phenomena at length scales larger than the size of our cell, we repeated our calculations with in-plane strain, as described below. In the silica, all Si atoms are 4-fold coordinated to oxygen, the mean Si−O−Si angle is 144°, the mean O−Si−O angle is 109.5°, the average Si−O distance is 1.63 Å, and the average Si−Si distance is 3.06 Å. These mean values are all very similar to those found in many silica polymorphs.37 Still, in this case they have a nonnegligible variance, as the Si−Si and Si−O radial distribution functions indeed do not display clear peaks as in a crystal, but rather broad peaks more similar to an amorphous phase (not shown). On the other side, the simulation cell is a (2 × 2) supercell of the anatase (101) surface, and therefore the silica slab is forced by the choice of the cell to be commensurate to that of anatase (101) with this periodicity. Oxygen is bound to two silicon atoms in bulk silica and to three Ti atoms in bulk titania. At the interface, intermediate average coordination is found, with oxygen atoms bound to either a Si and a Ti atom or a Si atom and two Ti atoms. We further investigated the stability of the system by performing a high-temperature (1000 K) simulation at constant particle number N, volume V, and energy E (NVE), starting from the 0 K optimized structure described above. The 0.2 fs time step used was sufficiently small to guarantee a very good energy conservation at such temperature. In the 0.5 ps simulated, we observed no geometrical instability or local rearrangement. We did not
Figure 1. Model of interface between titania and silica. Gray balls: titanium; yellow balls: silicon; red balls: oxygen; white balls: hydrogen.
attempt to explore the configuration space further as the main goal of this work is the study of the electronic structure of the interface with a sctructural model containing the most likely structural motifs. The local density of states reveals the special role played by specific interface states (Figure 2). The 2p states of the oxygen atoms bound to one Si atom and one Ti atom at the interface (Ti−O−Si) give a large contribution near the edge of the valence band, whereas the states of oxygen atoms bound to one Si atom and two Ti atoms (2Ti−O−Si) are located ∼1 eV below, so that their edge is deeper than that of O in bulk TiO2 (Figure 2a). This suggests that, upon photoexcitation, the valence holes might have a preference for the (Ti−O−Si) links. To better understand the nature and location of states at the valence band edge, we have analyzed the charge density of the state close to the valence band edge. The state with the highest energy is shown in Figure 3. Figure 3a shows an isosurface of the electron density of the highest energy occupied state of the system, and Figure 3b shows the electron density averaged upon planes parallel to the titania/silica interface. The second highest state is almost degenerate with the first one and displays 11063
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Figure 2. Total and projected density of electronic states (DOS) for the interface between A-TiO2 and amorphous silica, calculated with LDA. The solid black line is the total DOS. (a) Valence band at the interface titania/silica. The DOS projected on the 2p states of oxygen atoms is shown. Green dashed line: atom in the bulk TiO2; black dotted line: oxygen bound to one Si and one Ti; red dotted-double-dashed line: oxygen bound to one Si and two Ti. (b) Conduction band at the interface titania/silica. The DOS projected on the 3d states of titanium atoms is shown. Green dashed line: atom in the bulk; black dotted line: atom in the uppermost titania layer, participating in Ti−O−Si bonds; red dotted-double-dashed line: atom participating in a 2Ti−O−Si structure. The projected DOS have been multiplied by a factor 32 in the case of the conduction band and by a factor 64 in the case of the valence band to put them one the same scale as the total DOS.
robustness of these results with respect to interface strain, we expanded our model system with a uniform strain of 0.03 in both interface plane directions. The resulting projected density of states (see Figure S3, Supporting Information) confirms the trends earlier obtained, suggesting that our conclusions are fairly robust to strain state variations. Silica Monolayer. As a further step to model the effects of the coexistence of the two phases at a surface, we have built a model of an anatase (101) surface covered by a single monolayer of silica. Controlled growth of ultrathin (i.e., monolayer-thick) oxide films has become a widespread and intensively investigated technology in the past years,38−45 and angstrom-thick silica films were grown on titania by chemisorbing and oxidizing silicon-containing molecules.23 Moreover, protective oxide layers might be employed to protect TiO2 from corrosion in devices for solar water splitting.46 In this work two chemical terminations have been considered: full hydroxylation and hydrogen-free termination. In both cases, each Si atom is tetrahedrally coordinated to 4 O atoms, the SiO2 framework forming one 8-, one 6-, and one 4membered ring per surface cell. These are the most common rings in silica crystals and glasses.47,48 The geometry of the monolayer is very similar to that of bulk silica: on the hydroxylated surface shown in Figure 4 the average O−Si−O angle is 109.5°, the average Si−O distance is 1.68 Å, and the average Si−O−Si angle is 143°, as in many silica polymorphs.37 In the surface unit cell of our hydroxylated SiO2 monolayer system three Si atoms yield a (2Ti−O−Si) group, and the other three Si atoms bind to an OH group pointing toward the vacuum (Figure 4), while no (Ti−O−Si) link is present. Our predicted H atoms adsorption energy is 0.22 eV, calculated with respect to the hydrogen-free termination and the isolated hydrogen molecule. We have performed calculations also at LDA level to facilitate the comparison with the case of the bulk interface. With LDA, the average Si−O distance is 1.65 Å (1.68 Å in GGA), and the adsorption energy of hydrogen is 0.31 eV. Beside these quantitative differences, there are no qualitative differences between the two functionals for this system. To understand the process of hydroxylation, it would be necessary to consider also the adsorption of the OH− from the solution. In the present work we are however only interested in understanding the effect of the hydroxylation on the electronic states at the interface. To this aim it should be noted that the
Figure 3. The two uppermost states of the valence band for the bulk titania/silica interface. (a) Isosurface of the electron density of the uppermost state (green); gray balls: titanium; yellow balls: silicon; red balls: oxygen. (b) Planar average of the electron density of the uppermost state (solid black line) and of the second-highest state (red dashed line), in planes parallel to the titania/silica interface.
an analogous charge density distribution, symmetric with the one in Figure 3 but concentrated at the other interface. The third state from the top of the band is located about 0.2 eV from the first state and has a clear TiO2-bulk nature. The slow decay of the electron density of the topmost states into the slab is consistent with the small energy difference with respect to titania bulk states. In the conduction band, the 3d states of the Ti atoms participating in the (2Ti−O−Si) links give a large contribution near the band edge (Figure 2b). Still, the lowest states of the conduction band are located well within the titania slab, and thus conduction electrons may have a preference for bulk states. Overall, the projected density of states suggests that holes should be present at the interface. To investigate the 11064
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Figure 4. Model of hydroxylated silica monolayer from the side (a) and from above (b). Gray balls: titanium; yellow balls: silicon; red balls: oxygen; white balls: hydrogen.
Figure 5. Model of hydrogen-free silica monolayer from the side (a) and from above (b). Gray balls: titanium; yellow balls: silicon; red balls: oxygen.
structure presented here, with one OH group bound to a surface silicon, is typical of hydroxylated surfaces of bulk silica. In the hydrogen-free termination, three Si atoms participate in a (Ti−O−Si) group and the other three form a (2Ti−O−Si) group each, with no dangling bond pointing toward the vacuum (Figure 5). We next computed the total density of electronic states and its projection on selected atoms to investigate the effect of the silica monolayer and its hydroxylation state on the surface electronic properties. Figure 6 shows the electronic density of states (DOS) of the bare (101) anatase surface and of the SiO2 monolayer system (both hydroxylated and hydrogen-free), together with the DOS projected on bulk, interface, and surface atoms. In the bare anatase surface system both valence and conduction bands bend up in proximity of the surface, hosting undercoordinated oxygens. In particular, this would suggest that for the photoproduced holes,1 rather than for the conductions electrons, there is a greater tendency to be present at the interface (see Figure 7, in agreement with ref 18). The presence of a hydroxylated silica monolayer leads to an inversion of this band bending effect, so that the valence band edges at the phase boundary are positioned ∼0.5 eV lower than in the bulk TiO2. Based on this effect, electron holes will be predicted not to be present at the surface, as they are in pure anatase.1,18,49 The calculated downward shift of the TiO2 valence band is similar to the one obtained in the bulk TiO2/ SiO2 interface system, associated with (2Ti−O−Si) linkages. This reinforces the hypothesis that, in general, the relatively
high-coordinated bridging oxygen atoms located in (2Ti−O− Si) units offer relatively stable location for electrons at the interface between the two oxides. The behavior obtained for the anhydrous SiO2 monolayer system is quite different (Figure 6). In this case, the edge of the valence band is shifted upward while approaching the TiO2/ SiO2 interface. Namely, a significant upward energy shift is obtained for the DOS projected over the oxygens forming (Ti− O−Si) links. This is consistent with the lower coordination of these oxygens when compared with those forming (2Ti−O−Si) units and associated with downward energy shifts, as described above. Since in the anhydrous SiO2 monolayer system no H termination is available to saturate the O dangling bond at the interface with vacuum, a topological inversion of the SiO2 tetrahedral units is expected, ultimately yielding a higher density of linkage oxygens shared between the two oxides phases. In turn, this implies that fewer Ti atoms are available per bridging oxygen on the titania side of the interface, ultimately decreasing the ratio (2Ti−O−Si) over (Ti−O−Si) links. Since the oxygens at the SiO2/vacuum interface display a downward energy shift, the (Ti−O−Si) interface units are predicted to be the most stable hole sites in this system. Whether such holes would be available for chemical reactions taking place at the surface, after tunneling through the silica monolayer, remains an open question that goes beyond the scope of this work. 11065
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Figure 6. Total and projected density of electronic states (DOS) for the silica monolayer on A-TiO2(101), calculated with PBE. The solid black line is the total DOS. In the figures of the valence band, the DOS projected on the 2p states of oxygen atoms is shown; in the figures of the conduction bands, the DOS projected on the 3d states of titanium atoms is shown. Green dashed line: atom in the bulk. (a) Valence band at the interface titania/silica for the anhydrous monolayer. Black dotted line: oxygen bound to one Si and one Ti; black dot-dashed line: oxygen in a bridge position between two Ti atoms, and no bond to Si atoms. (b) Conduction band at the interface titania/silica for the anhydrous monolayer. Black dotted line: atom in the uppermost titania layer, participating in Ti−O−Si bonds; black dash-dotted line: atom in the uppermost titania layer, not participating in Ti−O−Si bonds. (c) Valence band at the interface titania/silica for the hydrated monolayer. Red dotted-double-dashed line: atom in the uppermost titania layer, participating in 2Ti−O−Si bonds; black dash-dotted line: atom in the uppermost titania layer, not participating in 2Ti−O−Si bonds. (d) Conduction band at the interface titania/silica for the hydrated monolayer. Red dotted-double-dashed line: atom in the uppermost titania layer, participating in 2Ti−O−Si bonds; black dash-dotted line: atom in the uppermost titania layer, not participating in 2Ti−O−Si bonds. (e) Valence band at the bare titania surface. Black dotted line: atom in the uppermost titania layer. (f) Conduction band at the bare titania surface. Black dotted line: atom in the uppermost titania layer. The projected DOS have been multiplied by a factor 30 in the case of the conduction band, by a factor 60 in the case of the valence band, to put them on the same scale as the total DOS.
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SUMMARY The TiO2/SiO2 system has been investigated by atomistic simulations based on density functional theory, in the two cases of a bulk interface and of a silica monolayer on a titania surface.
In the case of the TiO2/SiO2 interface, a strong connection between the bonding topology and the electronic density of states has been obtained in the calculations. In particular, we find that the formation of (2Ti−O−Si) links can be related to the downward bending of the valence band, while (Ti−O−Si) bonds induce an upward energy shift. The density of states in all these systems can thus be simply rationalized by considering their individual bonding topology. Our results suggest a tendency for holes to be present in atomically sharp TiO2/ SiO2 interfaces, both if the TiO2 phase is covered by a bulk SiO2 phase or by a single anhydrous SiO2 coating layer. In particular, using a single SiO2 coating layer, hole presence could be simply tuned by controlling the interface bonding topology via its surface chemical termination.
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ASSOCIATED CONTENT
S Supporting Information *
Figure 7. Schematic view of the band bending in the proximity of the surface of anatase. Both conduction and valence bands bend upward, driving holes to the surface and conduction electron toward the bulk.
Atomic coordinates and cell parameters for the bulk titania/ silica interface and for the silica monolayer at a titania surface; 11066
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more detailed versions of Figures 2 and 6, together with the projected density of states of a strained interface. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors acknowledge financial support by the European Commission under the EU-FP7-NMP grant 229205 “ADGLASS”. Computational resources were provided by Cineca and by the ICTP cluster Argo.
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