Highly efficient rutile TiO2 photocatalysts with single Cu(ii) and Fe(iii) surface catalytic sites

Faculty of Chemistry and Biochemistry, Ruh 44780 Bochum, Germany. E-mail: radim.be de; Fax: +49-731-5025409; Tel: +49-731-502 Department of Chemistry, University Colleg Institute of Chemical and Engineering Scie Research (A*STAR), 1 Pesek Road, 627833, Institute of Materials Research and Enginee Research (A*STAR), 3 Research Link, 11760 Faculty of Chemistry, Jagiellonian Universi EMAT, University of Antwerp, Groenenborge Matter and Materials Group, College of Sci Cook University, Townsville, 4811, Australia Institute of Electrochemistry, Ulm Univers Germany † Electronic supplementary information theoretical methods; XAS data; photo mixtures; UV-Vis spectra during 4-CP HAADF-STEM images and EELS spectra; stability measurements. See DOI: 10.1039 Cite this: J. Mater. Chem. A, 2016, 4, 3127


Introduction
Sunlight-driven heterogeneous photocatalysis utilizing low-cost materials is potentially one of the most attractive methods for decontamination of water or air from toxic organic pollutants. [1][2][3][4][5][6][7][8] However, real-life commercially viable applications of photocatalytic depollution are still rather scarce, due to insufficient performance stability and typically very low photocatalytic reaction rates. In terms of performance stability, the typical material of choice is titanium dioxide due to its excellent stability against photocorrosion, non-toxicity, low cost, and possibility for further functionalization. 3,5,7,9,10 Efforts to improve the photoactivity of TiO 2 have mainly focused on shiing the light absorption edge of pristine TiO 2 (3.2 eV for anatase, 3.0 eV for rutile; $390-410 nm) into the visible range by doping TiO 2 with metals or main group elements. 4 However, this approach has only rarely led to activity enhancements under solar irradiation, mainly because of diminished oxidizing power of photogenerated holes and due to enhanced recombination via intra-bandgap states introduced by doping. 4,[11][12][13][14][15][16][17] In this context, it is important to realize that even under UV light irradiation the quantum yields of organic pollutant degradation reactions at pristine TiO 2 are very low, typically only a few per cent. 18 This means that majority of charges photogenerated by UV light in TiO 2 are lost via recombination before they can induce redox reactions. Notably, it has long been suggested by Gerischer and Heller that the rate-limiting reaction in environmental photocatalysis is the reduction of oxygen by photogenerated electrons. 1,19 Indeed, this has been recently conrmed by kinetic studies using transient absorption spectroscopy which have shown that the reduction of dioxygen by photogenerated electrons is much slower ($ms timescale) than, for example, the oxidation of alcohols by photogenerated holes ($ns timescale). 20,21 This suggests that a very promising strategy for enhancing photodegradation rates at TiO 2 is to improve the kinetics of oxygen reduction at the photocatalyst surface by depositing a cocatalyst which would catalyze the transfer of photogenerated electrons to oxygen molecules. Faster channeling of photogenerated electrons from TiO 2 to oxygen molecules would diminish recombination and enhance charge separation (see Fig. 1). 22 The feasibility of this approach is well documented in the literature. For example, it is known that deposition of small amounts of platinum cocatalyst nanoparticles onto TiO 2 leads to enhanced photocatalytic efficiencies for pollutant degradation. 1,[23][24][25][26][27] Obviously, for large-scale applications cocatalysts based on abundant, non-noble materials are needed. Notably, Ohno et al. observed enhancement of photocatalytic degradation rates of gaseous acetaldehyde under both UV and visible light on rutile TiO 2 impregnated with Fe(III), Cu(II), Ni(II), and Cr(III) ions. 28 Based on double-beam photoacoustic spectroscopy measurements, the authors concluded that the transition metal ions improved the efficiency by acting as electron acceptors and as electron donors under UV and visible light, respectively. Later, Hashimoto et al. reported enhanced visible light activity in photocatalytic decomposition of isopropanol in the gas phase at rutile TiO 2 powders modied with small CuO x and FeO x clusters. [29][30][31][32] The enhanced photoactivity in the gas phase was ascribed to visible light-mediated direct optical charge transfer from the valence band of TiO 2 to energy levels in the cocatalyst clusters lying below the conduction band edge of TiO 2 , whereby the lower reducing power of visible-light photogenerated electrons towards oxygen was compensated by the ability of the cocatalyst to catalyze two-electron transfer to oxygen that occurs at more positive potentials (E 0 ¼ À0.16 V vs. NHE and +0.69 V vs. NHE for one-electron and two-electron reduction of oxygen, respectively). [29][30][31][33][34][35] Similarly, improved degradation rate of gaseous acetaldehyde under visible light irradiation was reported for N-doped TiO 2 loaded with copper ions by Morikawa et al. who suggested that the presence of Cu leads to prolonged lifetime of photogenerated charge carriers, 36 and for WO 3 photocatalysts impregnated with CuO by Sayama et al. who ascribed the activity enhancement to improved catalysis of oxygen reduction reaction. 37 In the present work, we focus on the deposition of redox cocatalysts for oxygen reduction onto TiO 2 in order to enhance the intrinsic UV light activity of TiO 2 in degradation of aqueous organic pollutants. We assume that already boosting the reaction rates under UV light irradiation alone due to improved catalysis of the rate-limiting oxygen reaction would yield photocatalysts for commercially viable applications in solar water decontamination. In a similar vein to the work of Ohno et al. 28 and Hashimoto et al., [29][30][31][32] we have recently impregnated rutile TiO 2 powders with small amounts of copper(II) or iron(III) nitrate. 38 Aer a mild heat treatment, such surface-modied rutile photocatalysts exhibited highly enhanced activity, as compared to pristine rutile TiO 2 , in photocatalytic degradation of 4-chlorophenol (4-CP) in aqueous phase under simulated solar light irradiation (l > 320 nm). We tentatively suggested that small CuO x and FeO x clusters were formed on the TiO 2 surface aer impregnation and drying, and that these small clusters allowed for improved catalysis of dioxygen reduction by photogenerated electrons, leading to enhanced photoactivity. 38 Herein we report our further detailed structural, spectroscopic, and theoretical investigations of these surface-modied rutile materials, and compare their photoactivity with rutile TiO 2 modied by conventional deposition of platinum nanoparticles. Most importantly, we provide conclusive experimental evidence for the presence of single Cu(II) and Fe(III) cocatalytic sites in our highly active photocatalysts, which differentiates them from conventional composites of TiO 2 with metal or metal oxide particles. [29][30][31][32]36,37,[39][40][41][42][43] The mechanistic aspects of the photoactivity enhancement at single ion-modied photocatalysts are discussed based on both theoretical DFT calculations and experimental (spectroscopic and photoelectrochemical) methods.

Photocatalytic activity
Highly active photocatalysts were prepared by simple impregnation of nanocrystalline rutile TiO 2 powders with very small amounts of copper(II) and iron(III) nitrate. The optimum amount (actual loading) of Cu and Fe leading to maximum photocatalytic degradation rates of a test organic pollutant (4-chlorophenol, 4-CP) was found to be very low: 0.12 wt% for Cu and 0.13 wt% for Fe. 38 For comparison, rutile TiO 2 decorated with Pt Fig. 1 Simplified scheme showing the concept of enhancing the photocatalytic degradation rates by deposition of cocatalysts for oxygen reduction: without cocatalyst the oxygen reduction is slow and the recombination of photogenerated electrons is fast (a); deposition of a cocatalyst enhances the rate of oxygen reduction, rendering the charge separation more efficient and the recombination slower (b). nanoparticles (optimum Pt loading of 0.34 wt%) was prepared by conventional photoreduction method [44][45][46] and served as a benchmark. Aer 3 hours of simulated solar irradiation (l > 320 nm) the TiO 2 (R)-Cu and TiO 2 (R)-Fe induced 4-CP degradation of 80% and 54%, respectively (Fig. 2a), a signicant enhancement as compared to the photoactivity of pristine rutile TiO 2 (25%). The benchmark platinized TiO 2 (R)-Pt material showed degradation of 73%. The highest initial degradation rate of 2.8 Â 10 À8 mol L À1 s À1 was observed for TiO 2 (R)-Cu (Fig. 2c), a value higher by a factor of 7 than the pristine rutile TiO 2 , and is even higher than for TiO 2 (R)-Pt (by a factor of 5.5 vs. pristine TiO 2 ). Furthermore, TiO 2 (R)-Fe also demonstrates an enhancement in degradation rate (by a factor of 4 vs. pristine TiO 2 ), although this is less enhancement than for TiO 2 (R)-Cu. Aer purging the suspension with argon, the initial degradation rates decreased drastically, conrming the crucial role of dissolved oxygen as an electron acceptor in photocatalytic degradation. Complete mineralization of 4-CP molecules to carbon dioxide, water and chloride ions was conrmed by monitoring the TOC (Total Organic Carbon) values during the photocatalytic reaction (Fig. 2b). Interestingly, the platinized TiO 2 (R)-Pt sample seems to be more effective in inducing complete mineralization than TiO 2 (R)-Cu, which can be ascribed to the well-known high activity of platinized TiO 2 in complete decarboxylation of carboxylic acids [44][45][46] which are important intermediates in the mineralization process of 4-CP. It should be noted that the photoactivity enhancement of modied samples is not the result of enhanced activity in the visible range. Though TiO 2 (R)-Cu and TiO 2 (R)-Fe show a very slightly yellowish tint to the naked eye, their fundamental optical absorption edge is the same as in case of pristine TiO 2 (Fig. 3). Accordingly, the photocatalytic degradation under visible light only (l > 455 nm) was found to be negligible for all samples. 38 Two things are noteworthy. Firstly, simple physical mixtures of rutile TiO 2 with CuO or Fe 2 O 3 (in amounts corresponding to the optimum loading) prepared by grinding do not induce any photoactivity enhancement (see ESI, Fig. S6 †). Secondly, while highly active TiO 2 (R)-Cu and TiO 2 (R)-Fe samples are obtained aer a mild drying at 120-150 C, aer calcination at high temperatures (450 C) the photoactivity is diminished drastically, though the Cu and Fe content is practically the same as in active samples. These results suggest that the chemical nature of Cu and Fe species at the surface of rutile TiO 2 is of crucial signicance for high photoactivity.

Structural characterization
Due to the low concentration of Cu and Fe species, both Raman spectra and X-ray diffractograms have shown only the typical pattern of rutile TiO 2 (ESI, Fig. S7 and S8 †). The primary crystallite sizes calculated using Scherrer's formula are 12-13 nm for all samples. High temperature treatment results in increase of rutile crystallite size to 20 nm and 15 nm for TiO 2 (R)-Cu (450 C) and TiO 2 (R)-Fe (450 C), respectively, which alone cannot  explain the decrease of activity aer calcination. We assumed that this is rather related to changes of the chemical nature of Cu and Fe species at the rutile surface.
In this context, it is intriguing to realize that the optimum loading (0.12 wt% Cu and 0.13 wt% Fe) in TiO 2 (R)-Cu and TiO 2 (R)-Fe is very low, indeed. Given the BET specic surface area of rutile TiO 2 (111 m 2 g À1 ), such loading corresponds to the surface density of 0.10 Cu atoms per nm 2 and 0.13 Fe atoms per nm 2 . This very low surface coverage let us to hypothesize that it could be single Cu(II) or Fe(III) sites which are present at the surface of rutile TiO 2 and are responsible for the enhanced photoactivity of the novel materials. This suspicion was further deepened by the fact that electron energy loss spectroscopy (EELS), which in the setup we used has sensitivity at the single atom level, could only show a very weak signal for Cu, while no signal for Fe could be detected (ESI, Fig. S9 †).
In order to gain further insight into the local chemical environment of Fe and Cu species and their oxidation states, we performed an X-ray absorption study. Fig. 4 presents the Cu Kedge XAS data (for the Fe data see ESI, Fig. S2 †). The beamline did not have sufficient energy resolution to resolve pre-edge data, but the lack of intensity indicates that there is not too much deviation from centrosymmetry. 47,48 The major peaks in the Fourier transform are labelled "1" to represent the 6 Cu-O distances @ 1.9-2.1Å, "2" to represent the Cu-Ti distances @ 2.8-3.00Å and "3" to represent the Cu-Ti distances at 3.4-3.6Å. As the data was only suitable for interpretation to k ¼ 10Å À1 no attempt was made to split the coordination spheres further. A t of the EXAFS data is given in the ESI (Fig. S1 †). The peaks in the Fourier transform of TiO 2 (R)-Cu both before and aer heating at 450 C are consistent overall with Cu(II) sitting "on" or "in" the rutile structure. There are, however, minor differences in the structure of the materials before and aer heating. Before heating there is a slight splitting of the coordination sphere, consistent with what one expects if the Cu(II) was decorating the surface of TiO 2 in a well distributed form. Aer heating the material becomes more integrated into the TiO 2 lattice, consistent with the slight shis of R values of the Fourier transform peaks. Similar effects, though less pronounced, are noted at the Fe edge (ESI, Fig. S3 †). Notably, the XAS data of bulk CuO x and a physical mixture of TiO 2 (R) with CuO show only two distinct peaks in the Fourier transform, and are thus clearly different from TiO 2 (R)-Cu. The XANES data of Cu are consistent with copper in oxidation state (II), the XANES data of Fe are consistent with Fe(III) with a high spin conguration.
In order to provide conclusive evidence for the presence of single surface Cu(II) and Fe(III) sites in our photocatalysts, an EPR study was performed. EPR is a highly sensitive spectroscopic technique which can give valuable information about the nature of d 9 Cu(II) and d 5 Fe(III) paramagnetic species in our materials. The most active TiO 2 (R)-Cu sample exhibits an anisotropic signal with hyperne structure due to I ¼ 3/2 of Cu(II) typical for isolated mononuclear slightly axially distorted octahedral Cu(II) complexes, 49 with resonance parameters A k ¼ 11.9 mT, g k ¼ 2.33 and g t ¼ 2.07 (Fig. 5a). This feature is a ngerprint of surface-bound single Cu(II) ions at rutile 50 or anatase TiO 2 , 49 or Cu(II) substituting lattice Ti(IV), 51,52 both assumed to be spectroscopically equivalent. The increase of Cu(II) concentration during impregnation yields photocatalytically inactive powders containing small CuO x clusters and nanoparticles. This is apparent from broadening of spectral lines due to long-range dipolar interactions between Cu(II) ions which results in gradual disappearance of the anisotropic hyperne structure (Fig. 5a). Following this trend, bulk CuO reference and the physical mixture of TiO 2 and CuO show only a very broad EPR signal (ESI, Fig. S10 †). The above results provide clear evidence for the presence of single isolated Cu(II) surface sites in highly active TiO 2 (R)-Cu.
Aer calcination of TiO 2 (R)-Cu at 450 C the EPR resonance parameters stay the same, but the hyperne structure is slightly less resolved (Fig. 5b). Importantly, an additional sharp signal with g ¼ 2.005 (denoted by an asterisk in Fig. 5b) appears, which is associated with formation of oxygen vacancies upon migration of Cu(II) ions from the surface to the sub-surface layer where it substitutes Ti(IV). 51 The same feature is observed upon heat treatment of copper complexes in the matrix of silicate glasses or on the surfaces of SnO 2 . 53,54 Further evidence for migration of Cu(II) from surface to the bulk aer heat treatment at 450 C comes from a simple experiment using an aqueous ammonia solution. Before the heat treatment the Cu(II) ions in TiO 2 (R)-Cu are easily accessible for reaction with ammonia to form a typical blue tetraamminecopper(II) complex, while aer the heat treatment at 450 C the sample does not show blue coloration upon addition of ammonia. In this context it is important to recall that the high-temperature treated sample is not photoactive. Based on our results, we conclude that this loss of activity is both because of migration of catalytically active surface Cu(II) ions to the subsurface layer and concomitant formation of oxygen vacancies, which both give rise to recombination centers.
A similar broadening of EPR spectra upon increasing the amount of Fe(III) is observed for TiO 2 (R)-Fe (Fig. 5c). Transition at g ¼ 4.31 (C) can be ascribed to the Fe(III) ions in orthorhombic sites at the surface of rutile TiO 2 . 55 Aer calcination at 450 C, the peaks A, B, D and E increased dramatically, suggesting that the Fe(III) ions migrated into the bulk. Indeed, the new EPR resonance transitions at g ¼ 8.02 (A), 5.54 (B), 3.47 (D) and 2.26 (E) have been unambiguously assigned to Fe(III) ions located in substitutional tetragonal sites of distorted axial symmetry where the Fe(III) have substituted Ti(IV) in the TiO 2 bulk lattice. 55,56 And, similar to the case of TiO 2 (R)-Cu, the most prominent new EPR signal arising aer the heat treatment at 450 C (g z 2.006, F) is related to substitutional Fe(III) ions accompanied by oxygen vacancies, 57 which render the sample photocatalytically inactive.

Theoretical calculations
We performed a set of DFT calculations to understand the inuence of single Cu(II) and Fe(III) ions on the electronic structure of the rutile TiO 2 surface in order to estimate the effects on photogenerated charge separation. The (110) surface of rutile was used in calculations as it is the most commonly observed surface facet. 58 First of all, we present the results for the effects of Fe-decoration. As for our previous work on Fe 2 O 3 decoration of anatase TiO 2 , 39 we investigated several different low energy adsorption sites on ideal rutile TiO 2 , the lowest of which is shown in Fig. 6a. The Fe(OH) 3 cluster binds to the twofold oxygen row of the rutile surface in such a way that the Fe(III) cation becomes octahedral. This is a very stable structure, with an exothermic binding energy of 3.64 eV. The Fe(OH) 3 cluster binds to two twofold oxygen atoms of the protruding row, and a surface threefold oxygen atom, resulting in a notable tilt. Furthermore, the Fe atom is not centrally located in the resultant FeO 6   The electronic structure of TiO 2 (R)-Fe is shown in Fig. 7a. We focus on the spin-down DOS, as this is where the totality of the Fe contribution to the DOS resides. As can be seen, the Fe atom, and indeed the cluster as a whole does not strongly modify the electronic structure at the band edges, implying that the primary charge separation properties of rutile are not affected. The bandgap of this composite system is at 1.80 eV the same as the ideal surface, this value is slightly smaller than our calculated bulk theoretical bandgap of 2.00 eV. The band edge character is the same as the ideal rutile surface. To further investigate the effects of Fe(III) decoration on charge separation we determined the atomic and electronic structure of the charged system. We relaxed the charged system to observe whether the electron localizes onto a specic site, resulting in the formation of an electron polaron. This will occur in an absorber-cocatalyst system when the conduction band (or LUMO) of the cocatalyst is lower than that of the light absorber. 43 Based on our analysis of the DOS of TiO 2 (R)-Fe we do not expect the extra electron to reside upon the Fe atom. There is a relaxation energy of 0.47 eV associated with the electron polaron, however upon inspection there is no strong structural rearrangement. Furthermore, from analysis of the DOS (see Fig. 7b), a state that does split off from the conduction band edge is not related to the Fe atom. Rather, this polaron state is associated with the Ti atoms that predominate at the conduction band edge (CBE). From analysis of the charge density difference, we observe that the excess electron localizes on a single Ti site in the middle of the slab, away from the surface. Specically, this is an octahedral sixfold coordinated Ti directly below a surface vefold coordinated Ti site, and exactly the same polaron localization as is observed for the ideal rutile TiO 2 -(110) surface. 59 This suggests that single Fe(III) decoration, as depicted in Fig. 6a, does not improve the thermodynamics of charge separation in ideal rutile TiO 2 .
In order to account for possible different modes of Fe(III) incorporation into the rutile surface, we further considered the effects of Fe cation addition to defective surfaces of rutile TiO 2 -(110), specically those where a single titanium surface vacancy is present. This defect was chosen as this provides a natural site for Fe(III) cations to reside, in comparison to other native point defects such as oxygen vacancies and titanium interstitials. Furthermore, under oxygen-rich conditions titanium vacancies (V Ti ) have a relatively low defect formation energy of 2.32 eV. 60 We denote this system TiO 2 (R)-Fe(V Ti ). We modelled the adsorption of a FeOH unit on the vacancy; the single OH functional group is required to obtain Fe(III). The structure of the nal relaxed geometry is shown in Fig. 6b. As can be seen, the titanium vacancy can  The electronic structure is shown as a DOS plot in Fig. 8. From inspection, it can be clearly seen that there is a significant iron presence in the spin-down channel near the CBE. More interesting, however, is that there is an acceptor state in the gap located above the Fermi level, specically 0.26 eV above the VBE and 1.54 eV below the CBE. Upon photoexcitation this implies that there would be a thermodynamic driving force for the photoelectron to transfer from the rutile crystal to the Fe(III) cation, improving charge separation. In order to investigate whether this is the case, we determined the geometry and thus the electronic structure of the electron polaron (Fig. 8b). The main difference in the DOS with respect to the uncharged system is that the unoccupied state above the Fermi level is now shied below the Fermi level, becoming occupied. The relaxation energy involved with this polaron formation is relatively tiny, at 0.022 eV, implying that at room temperature it will be easy for the electron to delocalize into the rutile lattice. Furthermore, although the charge density difference of the electron polaron shows that there is a concentration of electron density on the Fe(OH) site (see Fig. 9a), with closer examination we observe that there is a signicant minority of charge on the oxygen twofold coordinated rows, providing further evidence that the electron polaron is not strongly localized on the Fe(OH) site.
In contrast to the TiO 2 (R)-Fe system, the charge separation properties of TiO 2 (R)-Cu are much simpler to analyze. We present results for a single CuO cluster adsorbed on the rutile TiO 2 -(110) surface, with the geometry shown in Fig. 6c. There is a strong binding between the CuO cluster and the rutile TiO 2 -(110) surface, with an exothermic binding energy of 2.82 eV. The Cu(II) cation is fourfold coordinated, with Cu-O bonds of length 1.89Å, 1.93Å, 1.98Å, and 2.39Å. We were unable to stabilize octahedral species on the surface. We ascribe the missing atoms that enable octahedral coordination to the solvent, which is missing in our calculations where the surface is exposed to vacuum. Furthermore, the oxygen atom of the cluster forms a short bond of 1.79Å with a vefold coordinated titanium atom of the surface, closing a TiO 6 octahedra.
The electronic structure of TiO 2 (R)-Cu is shown in Fig. 10a. The Cu atom has a signicant presence on the CBE. This is primarily due to the Cu d-states. Further, we compare the position of the decorated rutile TiO 2 surface to the bare rutile surface, by comparison and alignment of the electrostatic potential in the vacuum region. 61 When the alignment is taken into account, the Cu-state is marginally ($0.1 eV) below the CBE of the ideal rutile surface. This would imply that any photoelectrons would transfer to the Cu(II), rather than stay in the rutile itself, although the thermodynamic driving force is quite weak.  As for the Fe(III)-decorated systems discussed above, we also determine the effects of the Cu(II) decorant on the photoelectron charge separation properties by investigating the thermodynamics for electron polaron formation. The addition of a single electron results in a substantial amount of reconstruction, with a reconstruction energy of 0.48 eV, very similar to the polaron in ideal rutile. As can be readily observed from inspection of the DOS, see Fig. 10b, the Cu-derived state is strongly stabilized by injection of the additional electron, with the state dropping by 1.0 eV and becoming occupied. This is related to the closure of the electronic state that is half-lled by the spare d-electron of the Cu atom. Furthermore, from calculation of the charge density difference we can directly determine where the excess electron, e.g. the photoelectron, resides. This photoelectron is strongly localized on the Cu atom, see Fig. 9b. In other words, Cu(II) decorants do aid charge separation by providing a thermodynamic trap, with a strong localization energy, for photoelectrons.
To summarize, we have shown that both Fe(III) and Cu(II) might improve the primary (thermodynamic) charge separation properties of rutile TiO 2 . However, the mechanism for charge separation is different for the two cations. For TiO 2 (R)-Fe, separation of the photoelectron from the rutile crystal does not occur for decoration by Fe(III), but for surface implantation. This therefore requires the presence of titanium vacancies, which are going to be a minority presence in any rutile sample. Additionally, the relaxation energy for electron polaron formation is very low at $0.02 eV, which may be overcome due to thermal uctuations. In contrast, for TiO 2 (R)-Cu, separation of the photoelectron from the rutile crystal does occur for decoration by Cu(II). These decorated systems also strongly localize the photoelectron at the surface site with a high relaxation energy, potentially improving the kinetics of subsequent oxygen reduction reaction. From our calculations there are clearly different charge separation mechanisms for the two metal cations.

Mechanistic investigations
Notably, the DFT calculations corroborate the experimental results showing that the optical absorption of TiO 2 (R)-Cu and TiO 2 (R)-Fe is practically the same as for pristine TiO 2 , $3.0 eV (see Fig. 3). This is important since the redox cocatalysts at the surface of composite photocatalysts (see Fig. 1b) should not parasitically absorb light and diminish thus the light harvesting by the TiO 2 light absorber. Clearly, single metal ion sites are sufficient to positively inuence the charge separation, whereby blocking of the light by cocatalyst is avoided since an extended lattice structure such as in metal oxide particles, does not develop. The positive effect of decoration of TiO 2 with single Cu(II) and Fe(III) sites on electron-hole separation is evidenced also by photoluminescence (PL) measurements (see ESI; Fig. S11 †). The PL intensity of both TiO 2 (R)-Cu and TiO 2 (R)-Fe is signicantly quenched as compared to pristine rutile TiO 2 , suggesting diminished radiative recombination both in the band-to-band mode (2.7-3.0 eV) and in the range typically attributed to surface state-mediated recombination (2.1-2.7 eV). 62 Though both Cu(II) and Fe(III) improve the photocatalytic performance (Fig. 2), the DFT results indicate that the reason for the improvement might be different for TiO 2 (R)-Cu and TiO 2 (R)-Fe. In order to shed light on the detailed mechanism of the photocatalytic degradation of 4-CP the reaction was conducted under different conditions. The 4-CP degradation is completely suppressed for all materials in the presence of EDTA acting as a strongly adsorbing hole scavenger (Fig. 11a). This underlines the substantial role of the oxidative pathway (holes and/or hydroxyl radicals) in the degradation reaction. In argonpurged suspension (see Fig. 2c) the photoactivity of all materials decreases. This nding conrms the essential role of oxygen as a primary electron acceptor. Residual photoactivity under argon can arise from the hole oxidation while electrons reduce residual traces of oxygen, lattice Ti(IV) ions in a rutile lattice, or Cu(II) and Fe(III) in TiO 2 (R)-Cu and TiO 2 (R)-Fe.
Degradation curves in the presence of oxygen or an alternative electron acceptor, namely tetranitromethane under argon, are shown in Fig. 11b. Pristine TiO 2 (R) degrades 4-CP much more efficiently in the presence of tetranitromethane as an electron scavenger than with oxygen. This means that, at pristine rutile, tetranitromethane scavenges the photogenerated electrons much faster as compared to dissolved oxygen, enabling the holes to oxidize efficiently the 4-CP molecules. This is in line with the reported reduction potential of tetranitromethane (+0.4 V vs. NHE) 63 which is more positive than in case of O 2 (À0.16 V vs. NHE). 35 In contrast, the degradation rates for TiO 2 (R)-Cu are practically the same in the presence of tetranitromethane and oxygen. This nding is very signicant. It conrms that the nature of reacting electrons in TiO 2 (R)-Cu is different from TiO 2 (R). The photogenerated electrons are apparently very efficiently trapped by Cu(II) sites, whereby the reactivity of Cu(II/I)-trapped electrons towards oxygen is enhanced, compensating kinetically the lower thermodynamic driving force for the reduction of oxygen as compared to the reduction of tetranitromethane. Moreover, the electron trapping at Cu(II) sites is clearly very fast, at least much faster than the reduction of tetranitromethane or oxygen by photoelectrons trapped by surface Ti(IV). Exactly as suggested by the DFT calculations, the charge separation in TiO 2 (R)-Cu will be dominated by the transfer of photogenerated electrons to surface Cu(II) ions. Rather different results were obtained for TiO 2 (R)-Fe which behaves quite similar to pristine TiO 2 (R), though the enhancement aer addition of tetranitromethane is not so pronounced. This suggests that either a signicant portion of reactive electrons in TiO 2 (R)-Fe might have a similar chemical nature (i.e., electrons trapped at Ti(IV/III) sites) to those in pristine TiO 2 (R), or the reactivity of electrons trapped at Fe(III/ II) surface sites towards tetranitromethane is much higher than for Cu(II/I)-trapped electrons. In line with our DFT calculations, the primary charge separation due to electron transfer to Fe(III) in TiO 2 (R)-Fe is less effective than in case of TiO 2 (R)-Cu. The degradation rates for TiO 2 (R)-Fe in the presence of oxygen are lower than for TiO 2 (R)-Cu, yet still higher than for pristine rutile TiO 2 . However, the mechanism of the enhancement at TiO 2 (R)-Fe seems to be different than in case of TiO 2 (R)-Cu.
In this context it is important to realize that dissolved oxygen not only serves as an electron acceptor, but can also become a source of various reactive species, like superoxide anion, hydrogen peroxide or hydroxyl radical, which can take an active part in the degradation process. Aer addition of tert-butanol, which is typically taken as a preferential scavenger of hydroxyl radicals, the photoactivity of TiO 2 (R)-Fe hardly changed and that of TiO 2 (R)-Cu dropped only slightly (Fig. 11c). It should be noted that tert-butanol does not adsorb strongly onto TiO 2 and therefore efficiently scavenges free hydroxyl radicals (cOH f ) but not surface-bound hydroxyl radicals (cOH s ). 64 Moreover, it has been reported that, in contrast to anatase where cOH f forms efficiently, at rutile TiO 2 mainly cOH s is produced. 64 Our results therefore indicate that free cOH f radicals do not play any signicant role in the enhancement of photocatalytic degradation rates of 4-CP molecules. However, the possibility that, apart from holes, also surface-bound cOH s radicals play some role cannot be completely ruled out. As a next step, we investigated the formation of hydroxyl radicals quantitatively. Fig. S12 (ESI †) shows uorescence spectra of hydroxyterephthalic acid formed upon irradiation of the material suspensions in a terephthalic acid solution under the ambient conditions. The production of hydroxyl radicals for each sample is proportional to the formation of hydroxyterephthalic acid, and is linear with time (see ESI; Fig. S13 †). 65 No correlation between hydroxyl radical production and photoactivity was found. The most active material, TiO 2 (R)-Cu, exhibits the lowest hydroxyl radical production, even lower than pristine TiO 2 (R) by a factor of 2. This points to a minor role of the hydroxyl radicals in the photooxidation mechanism at TiO 2 (R)-Cu. In contrast, TiO 2 (R)-Fe shows the highest rate of hydroxyl radical formation, which is higher by a factor of 2 than in the case of pristine rutile. Since the chemical nature of holes in all samples should be the same, we assume that this enhanced formation of hydroxyl radicals at TiO 2 (R)-Fe has its origin in the reductive pathway (initiated by reduction of oxygen). H 2 O 2 can be formed by a two-electron reduction of dioxygen catalyzed by Fe(III) sites. The H 2 O 2 molecules formed can be converted to hydroxyl radicals by further reduction by photogenerated electrons. Furthermore, Fenton-type reactions might be at play, which would involve reaction of Fe(II) and/or Fe(III) with H 2 O 2 under formation of highly oxidizing hydroxyl radicals (cOH) or hydroperoxyl radicals (HOOc), respectively. At any rate, apart from the quenching of PL mentioned above, the enhanced formation of (surfacebound) hydroxyl radicals is at present the only prominent feature of TiO 2 (R)-Fe which helps us to understand its enhanced photoactivity as compared to pristine rutile.
As it is known that the solubility of rst-row transition metal cations increases upon reduction, we investigated also the operational stability of our materials during four successive photocatalytic degradation experiments (ESI, Fig. S14 †). The activity of all modied samples, including TiO 2 (R)-Pt, decreased by $50% during four cycles, however still remaining higher than in case of pristine TiO 2 (R). As platinum clusters are not expected to undergo reductive photocorrosion, we attribute the decrease in activity of all samples to accumulation of decomposition intermediates at the surface of photocatalysts or to partial loss of photocatalyst during ltration aer each cycle. These problems can be overcome by optimization of operational parameters and reactor design.
Finally, we performed a set of photopotential decay measurements which are a powerful tool for directly probing the dynamics of photogenerated electrons, including the kinetics of their reaction with dioxygen. 66,67 During prolonged illumination under open-circuit conditions, the photoelectrons accumulate in the TiO 2 and shi the quasi-Fermi level of the electrode to negative potentials, until a steady state is achieved. Aer switching off the light, the open-circuit potential starts to decay. Under our experimental conditions, the rate of the decay depends on the concentration of accumulated charges and on the rate constants for two processes: the electron-hole recombination, and the reaction of electrons with dioxygen dissolved in the solution. In the presence of oxygen both processes are at work (Fig. 12a), whereas in the absence of oxygen the recombination process is predominant and is chiey responsible for the potential decay (Fig. 12b). A decay curve obtained by subtracting these two curves can serve as an indicator for the kinetics of dioxygen reduction (Fig. 12c). Our measurements clearly conrm the importance of fast dioxygen reduction for achieving high photocatalytic degradation rates. In the presence of oxygen the fastest potential decays are observed for the most photoactive materials, TiO 2 (R)-Cu and TiO 2 (R)-Pt. The fast decay is clearly due to enhanced rate of dioxygen reduction at TiO 2 (R)-Cu and TiO 2 (R)-Pt, as exemplied in Fig. 12c. In other words, Cu(II) sites act as efficient catalyst for O 2 reduction in a similar way as Pt particles. For TiO 2 (R)-Fe the situation is very different, the decay curves are similar to TiO 2 (R). This again conrms that the mechanism of the photocatalytic rate enhancement at TiO 2 (R)-Fe is distinct from TiO 2 (R)-Cu. Since the kinetics of primary O 2 reduction at TiO 2 (R)-Fe is apparently not improved as compared to TiO 2 (R), the enhancement is most probably due to new catalytic pathways to H 2 O 2 and hydroxyl radicals, opened by the presence of Fe(III) catalytic sites, as we discussed above.

Conclusions
Our experimental results combined with theoretical calculations demonstrate that single ion catalytic sites at the surface of TiO 2 photocatalysts are sufficient to considerably enhance the rate of photocatalytic decomposition of organic pollutants in water. The exact mechanism of the photoactivity enhancement can differ depending on the nature of the cocatalyst. For example, Cu(II)decorated rutile photocatalysts exhibit fast transfer of photogenerated electrons to Cu(II/I) sites, followed by enhanced catalysis of dioxygen reduction, resulting in improved charge separation and higher photocatalytic degradation rates. On the other hand, at Fe(III)-modied rutile the rate of dioxygen reduction is not improved, and the photocatalytic enhancement is attributed to higher production of highly oxidizing hydroxyl radicals produced by alternative oxygen reduction pathways opened by the presence of catalytic Fe(III/II) sites. Importantly, we have shown that excessive heating (at 450 C) of initially highly active photocatalysts leads to their deactivation due to migration of catalytically active Cu(II) and Fe(III) ions from TiO 2 surface to the bulk, which is accompanied by formation of oxygen vacancies. In terms of light harvesting, single-site-modied photocatalysts capitalize on the intrinsic UV light absorption by TiO 2 , whereby the isolated nature of surface cocatalytic sites guarantees negligible losses due to parasitic light absorption by the cocatalyst. The improved photocatalytic performance is chiey due to the electronic and redox properties of single ion sites, enhancing the charge separation, catalyzing "dark" redox reactions at the interface, and thus improving the typically very low quantum yields which represent the major bottleneck in environmental photocatalysis. These features make this type of materials distinct from more conventional visible light-active modied TiO 2 (ref. 29, 30 and 36) or TiO 2 composites with heterojunction structure, [39][40][41][42][43] and also from "single site photocatalysts" based on light-absorbing metal ion species dispersed on the surface of zeolites or silica. [68][69][70] As the photocatalytic activity of most photocatalysts is known to be highly substrate-specic 6,71 and depending also on a complex interplay of many material properties (crystallinity, porosity, surface area, relative amounts of specic crystal facets, etc.), 6 the demonstrated variety of mechanisms of photoactivity enhancement at single site catalyst-modied photocatalysts holds promise for developing many novel, tailored photocatalysts for various applications. Such efforts may also go beyond using TiO 2 as light absorber and include photocatalytic transformations other than aerobic degradation of organic pollutants.