Single-Molecule Vibrational Spectroscopy of H2O on Anatase TiO2

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Single-Molecule Vibrational Spectroscopy of HO on Anatase TiO (101) Christian Dette, Miguel A. Perez Osorio, Shai Mangel, Feliciano Giustino, Soon Jung Jung, and Klaus Kern J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b10379 • Publication Date (Web): 27 Dec 2016 Downloaded from http://pubs.acs.org on December 30, 2016

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The Journal of Physical Chemistry

Single-Molecule Vibrational Spectroscopy of H2O on Anatase TiO2 (101) Christian Dette†, Miguel A. Perez Osorio‡, Shai Mangel†, Feliciano Giustino‡, Soon Jung Jung†,*, and Klaus Kern†,§ †

Max Planck Institute for Solid State Research, Heisenbergstrasse 1, 70569 Stuttgart (Germany). Department of Materials, University of Oxford, Parks Road, Oxford OX1 3PH (United Kingdom). § Institut de Physique, École Polytechnique Fédérale de Lausanne, 1015 Lausanne (Switzerland). ‡

Supporting Information Placeholder is recorded as a step in the dI/dV or as a peak in the d2I/dV2 spectrum. This technique was first demonstrated in a pioneering experiment with single acetylene molecules on the Cu(100) surface10 and since then has been very successfully applied to explore single molecule chemistry on metal surfaces.11,12 For semiconductor surfaces, recording of STM-IETS is challenging due to the absence of states around the Fermi level (EF). To our knowledge, no successful experiment has been reported thus far. By using a highly n-doped naturally grown TiO2 anatase (101) crystal, we demonstrate in the present letter that we have a finite Fermi surface, which allows detecting vibrational modes of molecular and dissociated water on the anatase surface. These findings can potentially be transferred to different molecular/semiconductor systems, provided the semiconducting surface has a non-negligible density of states around EF.

ABSTRACT: Atomic level understanding of water on titanium dioxide (TiO2), in particular the mineral form anatase, is exceptionally important, considering its practical applications including photocatalytic water-splitting, photo-induced hydrophilicity and water purification.1-3 Although much effort has been devoted to the study of water adsorption and dissociation on anatase, investigating the fundamental steps of single water splitting remains challenging. These challenges arise from the difficulty of chemical identification of single molecular water and its dissociation products. Here, we have unequivocally identified single water molecules and hydroxyls on the TiO2 anatase (101) surface using scanning tunneling microscopy (STM) in combination with inelastic tunneling spectroscopy (IETS). Labelling of single molecules on the semiconductor surface was confirmed by demonstrating the isotope shift of the vibrational signatures and by first-principles density functional theory (DFT). The chemical identification of individual water molecules on anatase opens a direct path to follow the water splitting process on the single molecule level.

2. Methods The sample preparation of the natural grown crystal to obtain a clean TiO2 anatase (101) surface can be found in the method section of our previous publications.13,14 For the deposition, the sample was cooled down to 120 K and exposed to water (H2O or D2O), which was cleaned by freeze pumping cycles beforehand, to allow molecular adsorption5 at a chamber pressure of 10-9 mbar for 10 seconds. This preparation leads to the formation of thin ice covering the anatase surface (Supplementary Figure S2). In a second step, the sample was heated to room temperature resulting in single random molecular adsorption as shown in Figure 1a. These conditions favor random molecular distribution with only a limited number of molecular aggregates observed on terraces or step edges. The experiments were carried out in our homebuilt 5K STM using constant-current mode with a typical bias voltage of 1 V and tunneling currents between 0.05 to 1 nA. The tips used were electrochemical etched Pt-Ir tips bought from Agilent Technologies (N9801A), USA. For the IETS spectra, we recorded the differential conductance signal (dI/dV) with a set point current and bias of 100 pA and +0.5 V, respectively. The lock-in amplifier modulated the bias at a frequency of 420 Hz above the cut-off frequency of the feedback loop with a modulation voltage of 7 mV and an integration time of 300 ms. The dI/dV signal was then further differentiated to obtain the d2I/dV2 spectra. Due to the low contribution of the inelastic tunneling to the overall current (~ 0.1%)15 the obtained STM-IETS peaks of water on anatase are relatively weak. To get a reasonable signal-to-noise ratio, each displayed tun-

1. Introduction Substantial effort has been devoted to understand the interaction of H2O with TiO2 using an arsenal of spectroscopic tools such as photoelectron spectroscopy (PES), infrared spectroscopy (IRS) and temperature programmed desorption (TPD) in conjunction with DFT calculations.4-7 In particular, the physical and chemical properties of oxygen vacancies and their effect on surface reactions have been intensively studied.4 In general, these spectroscopic techniques detect and average signals obtained over large areas (≥ 100 x 100 nm2). To understand the elementary steps of water splitting, particularly the role of photogenerated holes and electrons, the direct probing of adsorbed species at the atomic scale is essential, for which STM has been proven to be an ideal tool. However, the key components of water splitting, H2O and OH, appear very similar in STM topography rendering the discrimination of the individual species difficult.8,9 Although the different behavior of H2O and OH in electric fields were used as a tentative assignment,9 no clear chemical identification was given so far. This ambiguity remains one of the biggest obstacles to achieve a microscopic understanding of water splitting on anatase. Here, we address this challenge by measuring molecular vibrations exploiting STM-IETS to label individual molecules with chemical sensitivity. In STM-IETS, the tip is fixed at a position over the surface and the bias voltage is swept to record a dI/dV curve using lock-in techniques. Once the applied bias voltage exceeds the energy of a molecular vibration, an additional tunnel channel opens increasing the conductance of the tip-sample junction. This

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neling spectra was obtained over a time period of 15-30 minutes. Therefore, we used an integration time of 300 ms, 512 points, and recorded 5-10 individual spectra, which were subsequently averaged. Due to the fact that the absolute values of the d2I/dV2 signal are different for spectra taken on the molecule and on the substrate, we normalized the spectra between 0 and 1. To exclude tip effects, we subtracted the substrate tunneling spectrum obtained directly before and after the individual measurements as displayed in Figures 1 and 2.16 As a consequence of the normalization, the subtraction can in some cases result in a non-flat baseline for the difference spectrum. However, no additional baseline correction was performed in this study to avoid artificial modification of data. Note that the IETS spectra were performed hundreds of times on different molecules, thus proving reproducibility. Computational details. We investigated the structural, electronic and vibrational properties of molecular and dissociated water, and the reaction pathway of water dissociation by first-principles calculations. The structural and electronic properties were investigated using density functional theory (DFT) and Car-Parinello method (CP); the vibrational properties were studied with the density functional perturbation theory (DFPT); and the reaction pathway of water dissociation was investigated with the nudged elastic band method (NEB). All the calculations were performed using the Quantum Espresso package.17 We used the Perdew-Burke-Ernzerhof exchange-correlation functional.18 Only valence electrons were explicitly described, including the Ti 3s and Ti 3d semicore states. The core-valence interaction was taken into account by means of ultrasoft pseudopotentials.19 The wavefunctions and charge density were described by plane-waves basis sets with energy cutoff of 35 and 140 Ry, respectively. The first Brillouin zone was sampled using a 2x2x1 Monkhorst-Pack k-point grid. The Ti 3d states were described by including Hubbard-like corrections using the simplified rotationally-invariant formulation of Ref. 20. The optimized structures, the calculated STM images and dissociation barrier were obtained using the Hubbard parameter U=3.5 eV.13,21 This value is very close to the Hubbard parameter determined from first principles using linear response calculations.22 All the structures are optimized until the largest force on the atoms is less than 0.04 eV/Å. The STM images were calculated within the Tersoff-Hamann approximation23 at constant current, by considering electronic states within 0.7 eV from the bottom of the conduction band. In order to investigate the isotopic effect of D2O, we calculated the normal mode frequencies with the mass of deuterium. To calculate the infrared intensities of the vibrational normal modes, we computed the Born effective charge tensors, using the modern theory of polarization.24 To investigate the effect of an applied electric field on the H2O molecule and the OH group absorbed on the TiO2, we performed molecular dynamics simulations with these systems in presence of an applied electric field perpendicular to the surface. A finite electric field was applied using the Berry phase formula25, with a magnitude of 5x109 V/m. This value is reasonable considering that typical bias values (1-3 V) in STM experiments induce electric fields between 1.6x109 and 1.0x1010 V/Å.26 We used the ground state of the single molecular and dissociated water as the initial and final states of the reaction pathway.

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further, as highlighted in high-resolution STM topographic images showing two individual molecules (Figs. 1b and d). The same images are represented in a different color code to facilitate distinguishing both species in Figures 1c and e. Whereas the top molecule (red squared) has a symmetric dumbbell shape, the lower molecule (green squared) is slightly asymmetric in size. The population of symmetric and asymmetric protrusions in several selected areas was manually counted and a ratio of approximately 7 to 3 was obtained (Supplementary Figure S4). In order to identify these two species, IETS was performed. Owing to the importance of the tip apex to the STM-IETS measurement and the frequent changes that can occur even during a measurement,16 we obtained the substrate spectra immediately prior and following to the spectra measured on the molecule. Figure 1f shows the IETS data obtained on the symmetric molecule from Figure 1b (i), on the pristine TiO2 anatase (101) surface (ii), and the corresponding difference spectrum (iii). The data reveal a clear peak around 191 mV which can be assigned to the molecular δ(H-O-H) bending mode.27,28 We have performed over 65 of these spectra resulting in a mean of 193.9 ± 12.3 mV. The detection of the bending mode unequivocally identifies the

Figure 1. H2O and OH adsorption on TiO2 anatase (101). (a) Emptystate STM image shows water adsorption on atomically resolved TiO2 (VS = 1 V, I = 1 nA, T = 5 K). (b-e) High-resolution STM images (linear scale) showing two isolated molecules of 1.4 Å in height with different features – a symmetric (red squared, b and c) and an asymmetric (green squared, (d) and (e)). (f) IET spectra (VS = 0.5 V, I = 0.1 nA) taken on the symmetric protrusion from (b) (i), pristine anatase (ii), and the difference spectrum (iii) show a distinct peak at 191 mV, which can be assigned to the δ(H-O-H) bending mode. The chemical fingerprint identifies the symmetric protrusion to be a H2O molecule. (g) In the case of the asymmetric protrusion (c) the bending mode is missing and this species is assigned as a dissociated water molecule (OH). (scalebars: (a) = 1 nm; (b) – (e) = 0.2 nm)

3. Results & Discussion Figure 1a shows an empty-state STM topograph of a TiO2 anatase (101) surface, covered with water molecules and hydroxyl species. Both species appear in dumbbell shape, 0.9 nm in length. These protrusions are situated above the Ti-O dimer row with their centers on top of a surface Ti5c atom. The STM features can be differentiated

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The Journal of Physical Chemistry features to be vibrational excitations.29 Since this shift also applies to the stretch modes, we are now able to designate the higher energy broad band due to reduced noise in this energy range. This allows us the assignment of this feature to be a superposition of different ν(O-D) stretch modes.27,28 IETS measurements on the asymmetric protrusion reveal again the absence of the bending mode at 146 mV but the broad band in the range of 220-350 mV is more pronounced with a maximum at 310 mV. Hence, the asymmetric protrusion is a hydroxyl OD (Figure 2b). The peak positions of the D2O and OD stretch modes agree well with our DFT calculations including the influence of hydrogen bonding (Supplementary Information). Considering the isotopic shift and the assignment of the ν(O-D) stretch modes here, we suggest that the broad feature in Figure 1 in the area above 400 mV might be attributed to different symmetric and asymmetric ν(O-H) stretch modes. The adsorption structure of H2O and dissociated water (OH+H) on TiO2 anatase (101) was calculated by means of DFT. Figures 3a and c show the corresponding results in the presence of an electric field generated by the STM tip (see SI for further details about the electric field effect). The oxygen of the H2O molecule forms a dative bond to a surface Ti5c whereas the oxygen of the OH forms a covalent bond (Figure 3a). Under influence of the electric field, both H2O and OH molecules are aligned perpendicular to the surface, with the H atoms pointing upward. The corresponding calculated STM images are presented in Figures 3b and d. Both protrusions are positioned on top of the Ti-O dimers centering on surface Ti5c atoms. The calculated STM images nicely reproduce the symmetry of molecular water and the asymmetry of hydroxyl observed in the experiment. Given the low diffusion barrier of hydrogen atoms, we calculated STM images for hydroxyls with and without hydrogen in the vicinity; the calculated STM images of OH were not affected by the position of adjacent H. The vibrational frequencies and infrared intensities were calculated using density functional perturbation theory (DFPT). We present the vibrational frequencies of the ground state, since the frequency shift induced by an applied electric field was negligible under the given conditions. For a single molecular H2O, the calculated frequency of the δ(H-O-H) bending mode is 195 meV (D2O: 143 meV), and the calculated frequency of the symmetric and asymmetric ν(O-H) stretching modes are 459 and 472 meV, respectively (D2O: 331 and 346 meV). In the case of the hydroxyl group, the ν(O-H) stretching mode is calculated to be 465 and 472 meV (OD: 338 and 343 meV). Furthermore, we calculated the effect of hydrogen bonding to a neighboring water molecule, which frequently occurs (Supplementary Table S1 and Figure S3). In FTIR, monolayer to multilayer coverages of water are measured. At these coverages, water can form hydrogen bonds with neighboring molecules. Indeed, we find a good agreement between the FTIR values of the water vibrational modes and our theory calculations of hydrogen bonded water complexes. The influence on the stretch modes is discussed in the supplementary information.

symmetric protrusion to be a single water molecule (red, 1b). The green spectrum in Figure 1g recorded on the asymmetric protrusion shows no peak around 190 mV but only a broad feature in the area above 400 mV, which is also present for the single water molecule. This broad feature is difficult to assign due to the high level of noise in this energy range. However, the study of deuterated water (see below) indicates that this broad feature is likely due to ν(O-H) stretch modes.27,28 This suggests that the asymmetric protrusion is a hydroxyl species (OH) resulting from water dissociation. To confirm the origin of the vibrational peaks, the isotopic shift of the molecular fingerprint was studied by substituting hydrogen with deuterium.29 For this, the TiO2 anatase (101) crystal was cleaned and subsequently D2O was adsorbed. Due to the similarity in the adsorption energies indicated by TPD data,5 our preparation procedure for deuterated water was the same as for H2O (Methods). The protrusions in the STM topographs have a similar size as in the case of the H2O molecules (0.9 nm) and appear again as symmetric or asymmetric dumbbell shapes. IETS measurements shown in Figure 2a on a symmetric molecule with a neighbor (red-squared) reveal a sharp peak at 146 mV and a broad feature in the range of 200-310 mV. Since the increased mass of deuterium induces a redshift of the vibrational peaks, we can again assign the lower energy peak to the δ(D-O-D) bending mode. This allows us to identify the symmetric protrusion in good agreement with previous results proving the origin of the inelastic

Figure 2. D2O and OD adsorption on TiO2 anatase (101). (a) IET spectra obtained on D2O with a neighboring water molecule (insert: STM image, i), pristine anatase (ii), and the difference spectrum (iii) reveal a distinct peak at 146 mV associated to the δ(D-O-D) bending mode. Furthermore, a broad feature in the range of 200-310 mV is attributed to the superposition of various symmetric and asymmetric stretch modes. (b) IETS spectra obtained on OD with a neighboring molecule (insert: OD molecule) showing the stretch mode in the range of 220-350 mV with a maximum at 310 mV but no peak at 146 mV. The difference in the peak positions of the D2O and OD stretch modes is in good agreement with our DFT calculations including the influence of hydrogen bonding (Supplementary Information). (scalebars: inserts in (a),(b) = 0.2 nm)

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Figure 4. 4 Calculated dissociation barrier for water on TiO2 anatase (101), diffusion barrier for the H atom on the surface, and ball-and-stick models of the H2O molecule, hydroxyl group, and the intermediate activated complexes. Upper curve: reaction pathway calculated with a slab containing 4 TiO2 layers, and by sampling the first Brillouin zone at the Gamma point. Lower curve: reaction pathway estimated from calculations of the dissociation energies between the configurations (i), (iii), and (v), using a larger slab (8 layers) and a finer (2x2x1) sampling of the Brillouin zone. In the lower curve the reaction barriers were estimated by scaling the barriers calculated in the upper curve via the ratios of the corresponding dissociation energies. The dissociation of (i) the H2O molecule to (iii) OH + H occurs via (ii) an intermediate activated complex, for which we estimated an activation energy Ea = 0.26 eV. This reaction is endothermic with an enthalpy ∆H= +0.16 eV. The subsequent diffusion of the H atom on the surface, from (iii) to (v), can take place by overcoming a potential barrier for which we estimate an activation energy Ea = 0.44 eV. The diffusion reaction is also endothermic with an enthalpy ∆H +0.15 eV. The dissociation and diffusion barriers in the upper curve were calculated using the nudged elastic band method (NEB) at the DFT+U level, with a Hubbard parameter of 3.5 eV (see Methods for further details and videos S1and S2 in SI, which corresponds to our NEB calculations of water dissociation and H atom diffusion).

Figure 3. 3 Ball-and-stick models and calculated STM images of molecular and dissociated water on TiO2. (a),(c) Atomistic model of the H2O molecule and dissociated water (OH + H) in presence of an applied electric field. (b),(d) their corresponding calculated STM images. The STM images were calculated at constant current by considering the electronic states within 0.7 eV from the conduction band edge. The magnitude of the applied electric field was 5x109 V/m corresponding to a tip bias voltage of 1-3 V. Units: meV

DFT

STM-IETS

Single H2O

195

191

H2O complex

199

201

Single D2O

143

146

D2O complex

146

147

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ATR-FTIR28

203

149

Table 1. Comparison of theoretical and experimental bending mode energies.

As shown in Figure 1, around 30 % of the observed protrusions appear asymmetric which can be assigned to hydroxyls resulting from water dissociation (Supplementary Figure S4). Since there was no illumination during our preparation procedure, we can exclude photo-induced dissociation. Hence, we consider the possibility of thermal dissociation by calculating the energy diagram of the water dissociation reaction (as shown in Figure 4). The first step of water dissociation is the formation of an intermediate complex (Figure 4(ii)), for which one hydrogen bond of the water molecule is weakened due to interaction with a neighboring surface O2c. Then, this hydrogen atom separates from the water molecule to create a H-O-Ti5c and H-O2c complex (Figure 4(iii)). The activation energy of the intermediate complex is around 0.26 eV. The dissociated state is energetically less favorable by 0.16 eV compared to the molecular water, suggesting the reaction to be endothermic. Owing to the relatively low height of the calculated dissociation barrier, however, the thermal dissociation of water at room temperature should be possible. The subsequent diffusion of the residual H atom to the nearest-neighbor O2c atom has an activation energy of 0.44 eV, which is low enough to occur at room temperature. Nevertheless,

these theoretical results should be taken as a qualitative description only because, as already pointed out in Ref. 30, we found that the energetics of the molecular and dissociated water is sensitive to parameters used in the calculations. For example, using a larger slab and a finer sampling of the Brillouin zone, the calculated energies decrease (Figure 4). Applying a simple rate equation using the energy values of the initial and final state of the water dissociation would result in a ratio of around 500:1 of molecular to dissociated water on the surface. The discrepancy to our observed ratio of 7:3 can be explained by the low diffusion barrier of the dissociated hydrogen atom on the anatase surface. This diffusion of the dissociated hydrogen atom suppresses the reverse reaction. Correspondingly, the expected population of OH does increase. More importantly, the dissociated state becomes more stable when neighboring molecules are present enabling hydrogen bonding.30 As we start with a high coverage of ice (Methods), all molecules are near each other, which favors dissociation. Therefore, we conclude that the observed hydroxyl species are thermally dissociated

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The Journal of Physical Chemistry (8) Stetsovych, O.; Todorovic, M.; Shimizu, T. K.; Moreno, C.; Ryan, J. W.; Leon, C. P.; Sagisaka, K.; Palomares, E.; Matolin, V.; Fujita, D.; et al., Atomic Species Identification at the (101) Anatase Surface by Simultaneous Scanning Tunnelling and Atomic Force Microscopy Nat Commun 2015, 2015 6, 7265(1-9). (9) Setvin, M.; Daniel, B.; Aschauer, U.; Hou, W.; Li, Y.-F.; Schmid, M.; Selloni, A.; Diebold, U., Identification of Adsorbed Molecules via STM Tip Manipulation: CO, H2O, and O2 on TiO2 Anatase (101) PCCP 2014, 2014 16, 21524-21530. (10) Stipe, B. C.; Rezaei, M. A.; Ho, W., Single-Molecule Vibrational Spectroscopy and Microscopy Science 1998, 1998 280, 1732-1735. (11) Kim, Y.; Komeda, T.; Kawai, M., Single-Molecule Reaction and Characterization by Vibrational Excitation Phys. Rev. Lett. 2002, 2002 89, 126104(1-4). (12) Komeda, T., Chemical Identification and Manipulation of Molecules by Vibrational Excitation via Inelastic Tunneling Process with Scanning Tunneling Microscopy Prog. Surf. Sci. 2005, 2005 78, 41-85. (13) Dette, C.; Perez-Osorio, M. A.; Kley, C. S.; Punke, P.; Patrick, C. E.; Jacobson, P.; Giustino, F.; Jung, S. J.; Kern, K., TiO2 Anatase with a Bandgap in the Visible Region Nano Lett. 2014, 2014 14, 6533-6538. (14) Kley, C. S.; Dette, C.; Rinke, G.; Patrick, C. E.; Čechal, J.; Jung, S. J.; Baur, M.; Dürr, M.; Rauschenbach, S.; Giustino, F.; et al., AtomicScale Observation of Multiconformational Binding and Energy Level Alignment of Ruthenium-Based Photosensitizers on TiO2 Anatase Nano Lett. 2014, 2014 14, 563-569. (15) Binnig, G.; Garcia, N.; Rohrer, H., Conductivity Sensitivity of Inelastic Scanning Tunneling Microscopy Phys. Rev. B 1985, 1985 32, 13361338. (16) Wahl, P.; Diekhöner, L.; Schneider, M. A.; Kern, K., Background Removal in Scanning Tunneling Spectroscopy of Single Atoms and Molecules on Metal Surfaces Rev. Sci. Instrum. 2008, 2008 79, 043104(1-4). (17) Paolo, G.; Stefano, B.; Nicola, B.; Matteo, C.; Roberto, C.; Carlo, C.; Davide, C.; Guido, L. C.; Matteo, C.; Ismaila, D.; et al., QUANTUM ESPRESSO: a Modular and Open-Source Software Project for Quantum Simulations of Materials J. Phys.: Condens. Matter 2009, 2009 21, 395502(1-19). (18) Perdew, J. P.; Burke, K.; Ernzerhof, M., Generalized Gradient Approximation Made Simple Phys. Rev. Lett. 1996, 1996 77, 3865-3868. (19) Vanderbilt, D., Soft Self-Consistent Pseudopotentials in a Generalized Eigenvalue Formalism Phys. Rev. B 1990, 1990 41, 7892-7895. (20) Cococcioni, M.; de Gironcoli, S., Linear Response Approach to the Calculation of the Effective Interaction Parameters in the LDAU Method Phys. Rev. B 2005, 2005 71, 035105(1-16). (21) Patrick, C. E.; Giustino, F., GW Quasiparticle Bandgaps of Anatase TiO2 Starting from DFT + U J. Phys.: Condens. Matter 2012, 2012 24, 202201(1-5). (22) Mattioli, G.; Filippone, F.; Alippi, P.; Amore Bonapasta, A., Ab Initio Study of the Electronic States Induced by Oxygen Vacancies in Rutile and Anatase TiO2 Phys. Rev. B 2008, 2008 78, 241201(1-4). (23) Tersoff, J.; Hamann, D. R., Theory of the Scanning Tunneling Microscope Phys. Rev. B 1985, 1985 31, 805-813. (24) Resta, R.; Vanderbilt, D. In Physics of Ferroelectrics: A Modern Perspective; Springer Berlin Heidelberg: Berlin, Heidelberg, 2007, 2007 p 31-68. (25) Umari, P.; Pasquarello, A., Ab Initio Molecular Dynamics in a Finite Homogeneous Electric Field Phys. Rev. Lett. 2002, 2002 89, 157602(1-4). (26) He, Y.; Tilocca, A.; Dulub, O.; Selloni, A.; Diebold, U., Local Ordering and Electronic Signatures of Submonolayer Water on Anatase TiO2(101) Nat. Mater. 2009, 2009 8, 585-589. (27) Maira, A. J.; Coronado, J. M.; Augugliaro, V.; Yeung, K. L.; Conesa, J. C.; Soria, J., Fourier Transform Infrared Study of the

water molecules. Moreover, we have performed experiments where the sample was never heated above 77 K. This preparation method only led to the adsorption of non-dissociated water molecules (manuscript in preparation). In this study, STM-IETS was employed to chemically identify single water molecules and hydroxyl species on the TiO2 anatase (101) surface. The identification of water and hydroxyl enables further studies on the fundamental processes of photocatalytic water splitting, including active sites on the anatase surface, the role of photogenerated charge carriers, and the details of charge transfer processes.

ASSOCIATED CONTENT Supporting Information Experimental Section, Influence of Hydrogen Bonding to the Vibration Modes, Calculation of the Electric Field Effect

AUTHOR INFORMATION Corresponding Author *[email protected]

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT M.A.P.O. and F.G. gratefully acknowledge funding from the Leverhulme Trust (Grant RL-2012-001), the UK Engineering and Physical Sciences Research Council (grants No. EP/J009857/1 and EP/M020517/1), and the Graphene Flagship (EU FP7 grant no. 604391), and supercomputing time at the University of Oxford Advanced Research Computing (ARC) facility (http://dx.doi.org/ 10.5281/zenodo.22558), the ARCHER UK National Supercomputing Service under the ‘AMSEC’ Leadership project and the ‘CTOA’ RAP project, and the Cartesius Dutch National Supercomputer under the PRACE DECI-13 project.

REFERENCES (1) Fujishima, A.; Honda, K., Electrochemical Photolysis of Water at a Semiconductor Electrode Nature 1972, 1972 238, 37-38. (2) Wang, R.; Hashimoto, K.; Fujishima, A.; Chikuni, M.; Kojima, E.; Kitamura, A.; Shimohigoshi, M.; Watanabe, T., Light-Induced Amphiphilic Surfaces Nature 1997, 1997 388, 431-432. (3) Mills, A.; Davies, R. H.; Worsley, D., Water Purification by Semiconductor Photocatalysis Chem. Soc. Rev. 1993, 1993 22, 417-425. (4) Oviedo, J.; Sánchez-de-Armas, R.; San Miguel, M. Á.; Sanz, J. F., Methanol and Water Dissociation on TiO2(110): The Role of Surface Oxygen J. Phys. Chem. C 2008, 2008 112, 17737-17740. (5) Herman, G. S.; Dohnálek, Z.; Ruzycki, N.; Diebold, U., Experimental Investigation of the Interaction of Water and Methanol with Anatase TiO2(101) J. Phys. Chem. B 2003, 2003 107, 2788-2795. (6) Kurtz, R. L.; Stock-Bauer, R.; Msdey, T. E.; Román, E.; De Segovia, J. L., Synchrotron Radiation Studies of H2O Adsorption on TiO2(110) Surf. Sci. 1989, 1989 218, 178-200. (7) Primet, M.; Pichat, P.; Mathieu, M. V., Infrared Study of the Surface of Titanium Dioxides. I. Hydroxyl Groups J. Phys. Chem. 1971, 1971 75, 1216-1220.

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Performance of Nanostructured TiO2 Particles for the Photocatalytic Oxidation of Gaseous Toluene J. Catal. 2001, 2001 202, 413-420. (28) Belhadj, H.; Hakki, A.; Robertson, P. K. J.; Bahnemann, D. W., In Situ ATR-FTIR Study of H2O and D2O Adsorption on TiO2 under UV Irradiation PCCP 2015, 2015 17, 22940-22946. (29) Nakamoto, K. In Handbook of Vibrational Spectroscopy; John Wiley & Sons, Ltd: 2006. 2006 (30) Patrick, C. E.; Giustino, F., Structure of a Water Monolayer on the Anatase TiO2 Surface Phys. Rev. Applied 2014, 2014 2, 014001(1-11).

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Single-Molecule Vibrational Spectroscopy of H2O on Anatase TiO2

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