Surfaces: A Scanning Tunneling Microscopy Study - American

Subscriber access provided by UNIVERSITY OF CONNECTICUT

Article

Water Adsorption on Cu2O(111) Surfaces – A Scanning Tunneling Microscopy Study Christoph Johannes Moeller, and Niklas Nilius J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b06996 • Publication Date (Web): 29 Aug 2017 Downloaded from http://pubs.acs.org on September 1, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Water Adsorption on Cu2O(111) Surfaces – A Scanning Tunneling Microscopy Study

Christoph Möller, Niklas Nilius* Carl von Ossietzky Universität, Institut für Physik, D-26111 Oldenburg, Germany

* Corresponding author: University of Oldenburg, [email protected], phone +49-441-798-3152

1 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT: The interaction of water with well-ordered Cu2O/Au(111) thin films has been explored with thermal desorption spectroscopy (TDS) and low-temperature scanning tunneling microscopy (STM). A TDS peak at ∼175 K indicates weak binding of the D2O monolayer to the Cu2O surface, while additional water condenses in disordered multilayers desorbing at 155 K. STM reveals flat water islands that nucleate along oxide step edges and grow up to a full monolayer at higher exposure. Molecules inside the islands take a local hexagonal order with 6 Å periodicity, while no long-range structures are formed. The observed adsorption behavior is compatible with a cation-free oxide termination that is Cu-deficient Cu2O(111) in the present case. For a stoichiometric film containing reactive CuCus ions, higher adsorption energies and stronger template effects for water would be expected, in contrast to the experimental findings.


Page 2 of 23

Page 3 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


INTRODUCTION The interaction of water with metal-oxide surfaces is key to many technologically relevant processes. With an increasing demand for renewable energies, hydrogen production via photochemical and electro-photochemical water splitting has gained enormous attention. TiO2 was identified as first electrode material for photon-induced water splitting.1 Since then, extensive research has been undertaken to advance the field, either by optimizing the TiO2 catalyst or moving to alternative materials, e.g. semiconducting oxides (ZnO, Ga2O3 and WO3).2,3,4 The main problem here is the correct adjustment of the materials band edges to promote both, the oxygen and hydrogen evolution reaction. The binding of water to oxide surfaces is yet of much broader importance and governs fundamental processes in heterogeneous catalysis, geochemistry, electrochemistry and atmospheric physics. In all these fields, a mechanistic understanding of the water-oxide system is required. However, insights at a truly molecular level was achieved mainly by theoretical approaches and experiments performed on few model oxides, such as TiO2, ZnO or FeOx.5,6,7,8 This research was complemented by numerous studies on less-defined powder samples, providing a phenomenological but no mechanistic view onto water-oxide coupling. The ability of water to interact with oxide surfaces is governed by two major principles, that is, binding of water-hydrogen and water-oxygen to the anions and cations in the oxide surface, respectively.9,10 The latter mechanism is typically more efficient, as it allows for covalent bonding besides the omnipresent hydrogen coupling. This was concluded, for example, from the reduced reactivity of oxygen-terminated FeO(111) with respect to metal-terminated Fe3O4(111) towards water.11 Other factors decide on dissociative versus associative binding schemes, among them the oxide basicity that controls electron transfer processes with water, the lattice mismatch between ad-layer and oxide support and the availability of O-vacancies in the surface.12,13,14 These factors successfully explained the dissociative water adsorption observed on rocksalt oxides 3 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

with large lattice parameter and high basicity (MgO→SrO),10,15 as well as the enhanced reactivity of open and defective in contrast to compact and stoichiometric surfaces. In this study, we examine the interaction of water with the Cu2O(111) surface, using a combination of scanning tunneling microscopy (STM) and thermal desorption spectroscopy (TDS). Cuprous oxide is of particular interest for photocatalytic water splitting, as demonstrated by numerous studies on Cu2O powder samples.16,17 In contrast to ZnO and TiO2, the material has a reduced band gap of 2.15 eV, which enables photo-activation not only by ultraviolet but also by visible light down to 600 nm wavelength. Cu2O is therefore able to harvest a much broader fraction of the solar spectrum, a property that gets amplified by the direct nature of the oxide band gap. Despite these advantages, fundamental studies of the Cu2O-water system are still scarce, both on the theoretical and experimental side. Density functional theory (DFT) calculations on stoichiometric Cu2O(111) find associative water binding to be favored over dissociative pathways, with the unsaturated Cucus surface ions serving as main adsorption sites.18,19 As expected, the open and polar Cu2O(100) and (110) facets are more reactive than the compact (111) plane.20 Experimentally, TDS has been used to study water adsorption to Cu2O(100), revealing associative and dissociative adsorption pathways for water dosing at 110 and 300K, respectively.21 Concerning STM, only a single experiment has been carried out to our knowledge, using the native copper oxide grown on Cu(111) as support.22 The study showed the formation of cyclic water clusters upon low-temperature deposition, the internal structure of which was analyzed with DFT calculations. We note that the native oxide of Cu(111) strongly deviates from the Cu2O(111) bulk phase due to its ultrathin and highly strained character.23 We employ a more realistic Cu2O(111) model system in this study, prepared by reactive Cu deposition on an Au(111) single crystal. The emerging Cu2O films are of high quality, thanks 4 ACS Paragon Plus Environment

Page 4 of 23

Page 5 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


to the good lattice match with the gold support (4.2% as compared to 18% in the case of Cu(111)).24 The water-oxide interaction is analyzed by means of STM and TDS. At low exposure, formation of small molecular clusters is found that grow into larger 2D islands at continuous dosing. The latter seem to be amorphous at first glance, however, detailed analysis reveals a short-range hexagonal order that resembles the atomic structure of Cu2O(111). With this information and the binding energy from TDS, we propose an adsorption scheme of water to the cuprous oxide surface.

EXPERIMENT The experiments have been performed with a custom-built beetle-type STM operated at liquid nitrogen temperature (85 K). The vacuum chamber (5×10-10 mbar base pressure) contains standard facilities for sample preparation and analysis, including setups for TDS and low-energy electron diffraction (LEED). The Cu2O(111) films are prepared by reactive Cu deposition in 5×10-6 mbar of oxygen onto a sputtered and annealed Au(111) surface. The film thickness is adjusted to 3-5 ML. Post-annealing to 600 K results in an ordering and smoothing of the over-layer that adopts a long-range hexagonal structure with ∼6 Å periodicity (Fig. 1a).24 Recent DFT calculations have suggested that this phase corresponds to stoichiometric Cu2O(111) and contains both, unsaturated Cucus and Ocus surface ions.25 As shown later, the model is not fully compatible with the experimental results of this study. The hexagonal Cu2O phase grows into ∼200 Å wide terraces, delimited by misfit-induced dislocation lines. Note that other oxide phases, such as Au〈112〉 and 〈110〉-oriented Cu-O stripe phases can be prepared at different conditions.24 For the sake of clarity, we disregard these structures in the following discussion.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

RESULTS AND DISCUSSION Water (D2O) is exposed via a pinhole doser pointing towards the freshly prepared Cu2O film. Room temperature dosing does not yield any measurable coverage, indicating negligible sticking at 300 K. Dosing at 100 K, on the other hand, quickly covers the surface with water, as inferred from thermal desorption spectra taken with 3 K/s heating rate (Fig. 1b). At low exposure, a faint desorption peak at ∼175 K is detected that saturates at about one monolayer (ML) and displays a faint high-temperature tail indicative for defect-related adsorption. At higher dosing, a second maximum develops at 155 K, which shows the known behavior of zero-order desorption, i.e. a common rising edge in coverage-dependent spectra. From the logarithmic increase of the desorption rate, an interaction energy of 0.45 eV is derived, the expected value for multilayers of amorphous solid water.26 The high temperature peak is then assigned to the interfacial D2O layer on Cu2O(111). Assuming first order desorption, the associated binding energy is estimated to ∼0.5 eV, only slightly above the bulk value for water cohesion. D2O adsorption to cuprous oxide seems thus to be weak.

Fig. 1: (a) STM overview image of hexagonal Cu2O(111) / Au(111) (1.5 V, 0.5 nA, 27 × 27 nm2). (b) Thermal desorption of D2O from the Cu2O surface after 100 K dosing and after flashing


Page 6 of 23

Page 7 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


to 160 K (solid and broken lines). The desorption rate is plotted logarithmically to highlight lowcoverage effects.

Not surprisingly, multilayer water films are inaccessible to STM due to their high mobility and low conductance. Careful annealing to 160 K produces however (sub-) monolayer films that are sufficiently stable to be probed with STM. Figure 2 shows an image series of the Cu2O(111) surface covered with increasing amounts of D2O. At the lowest coverage, isolated water clusters are discernable, the smallest ones being ∼10 Å in diameter and ∼2 Å in height (Fig 2a). Larger ad-clusters are of elliptical shape and might arise from coalescence of the smaller units. The inset in Fig. 2a shows an isolated cluster at higher resolution. Although no inner structure is resolved, it allows us to evaluate the binding position of the aggregate on the oxide film. Most of the water cluster are located either along domain boundaries, being inserted to remove misfit strain with the Au(111) support, or along step edges in the oxide film.24 We note that ultrasmall water clusters were observed on CuOx/Cu(111) films before and assigned to D2O hexamers by means of DFT.22 As we have no further information, we relate the D2O aggregates observed here to cyclic ad-clusters as well. With increasing coverage, irregularly shaped D2O islands develop on the Cu2O surface, as seen in Fig. 2b. In contrast to isolated clusters, the water islands preferentially nucleate at oxide step edges, both on their upper and lower side. Measured island heights are about 2.5 Å, independent of the tunneling bias and polarity. This value is higher than for isolated clusters, but smaller than a typical Cu2O step edge (3.5 Å). Water islands are readily distinguishable from the crystalline Cu2O surface by their amorphous appearance and ragged boundaries. The latter undergo gradual changes during scanning, indicating a certain mobility of the edge molecules that might be induced by the tip. Further water exposure leads to island growth, as shown in the STM 7 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

images in Figs. 2c and d that correspond to 40 and 75% nominal coverage, respectively. Evidently, complete D2O monolayers but no 3D structures may form in the 160 K annealing step, in agreement with the TDS data. The inner structure of the water islands is analyzed in the next paragraph.

Fig. 2: (a-d) STM images (1 V, 10 pA, 35 × 35 nm2) of the Cu2O(111) surface with D2O coverages of approximately 0.01, 0.2, 0.4 and 0.75 ML. The inset in (a) depicts a single water aggregate located on two domain boundaries in the oxide film (broken lines).

The water islands on Cu2O(111) appear as disordered arrays of bright dots with 1.0-2.5 Å height (Fig. 3). Such a broad height distribution is compatible with the coexistence of differently oriented water molecules. Although the molecular arrangement seems random at first glance, proper inspection discloses a certain short-range order. Three methods have been employed to analyze the STM images in this respect. The first one is based on a Fourier transformation of the 8 ACS Paragon Plus Environment

Page 8 of 23

Page 9 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


topographic data and works only for relatively large islands with no Cu2O exposed. A corresponding 2D power spectrum, produced from the island in Fig. 3a, features a prominent background due to structural disorder in the water layer (Fig. 3c). However, a faint hexagonal pattern becomes discernable as well. Interestingly, it has the same periodicity as bare Cu2O(111), as determined from the same data set. Apparently, the oxide surface exerts a weak, yet visible template effect on the molecular species. Further insight into the spatial distribution of water-related species comes from 2D paircorrelation functions that can be calculated for both, small and large islands. Also here, a hexagonal intensity pattern is observed, competing again with a strong disorder-related background (Fig. 3d). The signal-to-noise ratio gets improved when integrating over all azimuthal angles, thus producing a radial pair correlation function as shown in Fig. 3e. The first two maxima at 6 and 12 Å in the plot perfectly match the lattice parameter of Cu2O(111). The method thus confirms that D2O adsorption is influenced by the periodicity of the oxide underneath. Last but not least, we have explored possible D2O binding arrangements by extrapolating the well-defined lattice of exposed Cu2O regions to nearby water islands (Fig. 4a). Although the method does not yield atomic binding positions, because of the unknown course of dislocation lines below the island, it provides insight into the spatial correlation between ad-water and the oxide lattice. Again, a certain template effect is found, i.e. Cu2O(111) seems to expose suitable binding sites for D2O. However, the interaction is too weak to induce long-range order in the ad-layer, in contrast to many metal surfaces.27, 28



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 23

Fig. 3: High-resolution STM images of water islands on Cu2O(111) (a) (28 × 28 nm2) and (b) (15 × 15 nm2). (c) FFT patterns of clean and water-covered surface regions. The hexagonal lattice symmetry is visible in both cases, but competes with a prominent background in the water case. (d) 2D and (e) radial pair-correlation function of water-related maxima and the bare Cu2O surface, as calculated from the STM image in panel (a).

Information on the inner structure of water ad-islands is obtained from close-up STM images, as presented in Fig. 4. The measurements confirm the idea of a similar density of D2O molecules and Cu2O repeat units, as already suggested from overlapping maxima in the radial paircorrelation function. Despite the absence of long-range order, common D2O binding motifs are found in many cases, for instance the distorted six-membered rings highlighted in Figs. 4c and d. These arrangements demonstrate that the incoming molecules are able to adopt a local hexagonal order, but cannot homogenously cover the entire Cu2O(111) surface. Reasons for this are discussed in the final section of our paper. 10 ACS Paragon Plus Environment

Page 11 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Fig. 4: (a) Water island with superimposed Cu2O(111) lattice. After inserting a domain boundary between upper and lower half of the island, most molecules occupy similar lattice positions (4.5 × 4.5 nm2). (b) Boundaries between bare Cu2O (left), a water island (up) and an uncovered oxide terrace (right). The up-side of the oxide step is decorated with a chain of D2O molecules (5 × 5 nm2). (c) High-resolution image of a D2O island next to a bare oxide region (9 × 9 nm2). Whereas the upper panel depicts raw data, some characteristic D2O structures are highlighted in the lower one. (d) Two subsequent STM measurements, optimized for imaging the oxide surface (up) and the water islands (down) (7.5 × 7.5 nm2). Note the quasi hexagonal arrangement of molecules in the lower panel.

To rationalize the observed D2O binding behavior on Cu2O(111), we make use of recent DFT calculations on this topic.18,19 On stoichiometric Cu2O, the low-coordinated CuCus ions were identified as preferred binding sites for molecular water. Their dangling bond states form covalent bonds with the water oxygen with about 1.0 eV binding energy. Surface oxygen (Ocus), on 11 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 23

the other hand, is susceptible to hydrogen bonding, which leads however to considerably lower adsorption energies. In both cases, dissociative water adsorption is energetically unfavorable. Additional stabilization occurs in molecular ensembles via mutual hydrogen bonding.18 Saturation of all available Ocus and Cucus ions on the stoichiometric surface hereby results in particularly stable H2O arrangements, composed of interwoven six-membered water rings.22 The extra stabilization energy amounts to 0.35 eV per molecule in this case. Apart from stoichiometric Cu2O, a Cu-deficient termination has been considered, in which all Cucus ions are removed by generating a polar surface.29 The Cu-deficient termination generally features lower binding strength to water due to the absence of covalent bonds to the unsaturated Cu ions.18 This makes hydrogen bonding to Ocus the dominant interaction scheme and lowers the H2O adsorption energy to 0.5 eV, two times smaller than on stoichiometric Cu2O(111). Moreover, while the stoichiometric oxide forms an adsorption template for water, controlled by the hexagonally arranged Cucus ions, no ordering effect is expected on the Cu-deficient surface. On this basis, we interpret our water adsorption data on Cu2O thin films. TDS indicates weak D2O coupling to the oxide surface that hardly exceeds the cohesion of bulk water.26 According to the DFT calculations, this agrees well with hydrogen bonding to OCus ions, but is in conflict with the much stronger D2O-CuCus interactions. TDS thus provides a first hint for associative water binding to a Cu-deficient oxide surface. Further support comes from the absence of well-ordered adsorption patterns. Although STM measurements reveal a certain short-range order of the D2O molecules, matching the symmetry of the Cu2O lattice, the template effect is weak and only compatible with a water-oxide coupling mediated by OCus ions. Evidently, a Cudeficient surface agrees better with our experimental observation than a stoichiometric oxide with CuCus ions exposed.


Page 13 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


This conclusion disagrees however with an earlier interpretation, in which a stoichiometric surface was used to explain the experimental data.25 There, a specific contrast in empty-state STM images and a conductance resonance at 0.75 eV above EFermi were taken as fingerprints for the 4s dangling bond states associated to CuCus ions. The underlying hybrid-functional DFT calculations demonstrated however that the stability window of stoichiometric Cu2O(111) is limited to a small region at low oxygen chemical potentials,25 while Cu-deficient structures prevail at oxygen-rich conditions.18,25,29 Moreover, the CuCus ions were shown to become unstable at low film thickness, as the metal support partly compensates for the polarity of Cu-deficient surface terminations.25 The oxide films used in this study have apparently been prepared outside the stability window of stoichiometric Cu2O(111), either what the oxidation conditions or the film thickness concerns. First attempts to repeat the measurements with thicker, probably stoichiometric films failed so far, as no stable imaging of the weakly-bound D2O could be achieved. The ultimate experiment that is probing the water adsorption behavior on Cu2O films with and without CuCus can therefore not be presented at this stage. Let us finally add some thoughts on the packing density of water molecules on the Cu2O surface. Although the mean spacing of water-induced species was determined to 6 Å, the real packing density is likely higher. The typical interaction length of water in a hydrogen-bonded network is 2.3-2.8 Å; 0.8 Å for the O-H bond inside the molecule and 1.5-2.0 Å for the hydrogen bond to its neighbor.10,27 We take the formation of a hydrogen-bonded network for granted here, as D2O spontaneously assembles to extended islands and does not remain isolated, as on anatase TiO2 for example.30 Also, DFT calculations demonstrated that the rhombic 6×6 Å2 unit cell of Cu2O(111) is large enough to accommodate more than one molecule.18 Extra molecules might thus be present on the surface, but do not result in visible protrusions in the STM images. We explain this discrepancy with different orientations, hence different electronic properties of the 13 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 23

D2O molecules on Cu2O(111). Three standard adsorption geometries are discussed in the literature, the O-up, flat lying and H-up configuration.27,31 The latter is the preferred scenario for water bound to cationic species, while the former is compatible with an hydrogen bonding to oxide anions. Which conformer actually produces the visible features in STM cannot be decided without theoretical modelling, as this depends on topographic height as well as electronic nature of the adsorbates. The presence of additional water on the Cu2O surface is however presumed in order to rationalize the observed island formation. Note that even at a two times higher packing density, the D2O interaction length exceeds the optimal distance in a hydrogen-bonded network (3.1 Å versus 2.5-2.8 Å). This deviation might add to the inability of water to develop ordered adstructures on Cu2O(111). In contrast, crystalline water films are commonly observed on densepacked metal surfaces, where the binding constraints of the hydrogen networked are readily fulfilled by the substrate lattice.27

CONCLUSIONS Water exposure to Cu2O(111) thin films and subsequent annealing to 160 K was shown to produce extended D2O monolayer islands. Although disordered at first glance, closer inspection reveals a short-range order of the molecules with 6 Å spacing. Both, CuCus and OCus surface ions could be behind this template effect; however, the weakness of water-oxide binding better agrees with an Ocus-mediated interaction scheme. This implies that the explored Cu2O(111) surface is non-stoichiometric and bare of reactive CuCus species. The above binding scenario needs to be verified in future, preferably with infrared absorption or electron-energy-loss spectroscopy. We will further examine, whether weakly bound D2O can be photo-dissociated on the Cu2O surface, thereby exploring the potential of cuprous oxide for photocatalytic applications.


Page 15 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Acknowledgements: CM and NN are grateful for financial support via a DFG grant on ‘Photocatalytic processes probed at the atomic scale’. We thank J. Goniakowski and C. Noguera for extensive and fruitful discussions.

REFERENCES

(1)

Fujishima, A.; Honda, K. Electrochemical Photolysis of Water at a Semiconductor Electrode. Nature 1972, 238, 37–38.

(2)

Khan, S. U. M.; Al-Shahry, M.; Ingler, W. B. Efficient Photochemical Water Splitting by a Chemically Modified TiO2. Science 2002, 297, 2243–2245.

(3)

Maeda, K.; Teramura, K.; Lu, D.; Takata, T.; Saito, N.; Inoue, Y.; Domen, K. Photocatalyst Releasing Hydrogen from Water. Nature 2006, 440, 295.

(4)

Osterloh, F. E. Inorganic Materials as Catalysts for Photoelectrochemical Splitting of Water. Chem. Mater. 2008, 20, 35.

(5)

Bikondoa, O.; Pang, C. L.; Ithnin, R.; Muryn, C. A.; Onishi, H.; Thornton, G. Direct Visualization of Defect-Mediated Dissociation of Water on TiO2(110). Nat. Mater. 2006, 5, 189–192.

(6)

Meyer, B.; Marx, D.; Dulub, O.; Diebold, U.; Kunat, M.; Langenberg, D.; Wöll, C. Partial Dissociation of Water Leads to Stable Superstructures on the Surface of Zinc Oxide. Angew. Chemie - Int. Ed. 2004, 43, 6642–6645.

(7)

Parkinson, G. S. Iron Oxide Surfaces. Surf. Sci. Rep. 2016, 71, 272–365.

(8)

Lindsay R. Merte; Peng, G.; Bechstein, R.; Rieboldt, F.; Farberow, C. A.; Grabow, L. C.; Kudernatsch, W.; Wendt, S.; Lægsgaard, E.; Mavrikakis, M.; et al. Water-Mediated Proton Hopping. Science 2012, 336, 889–894.

(9)

Tocci, G.; Michaelides, A. Solvent-Induced Proton Hopping at a Water-Oxide Interface. J. Phys. Chem. C 2014, 5, 474.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(10) Zhao, X.; Shao, X.; Fujimori, Y.; Bhattacharya, S.; Ghiringhelli, L.; Scheffler, M.; Freund, H-J.; Sterrer, M.; Nilius, N.; Levchenko, S. Formation of Water Chains on CaO(001): What Drives the 1D Growth? J. Phys. Chem. Lett. 2015, 6, 1204–1208. (11) Joseph, Y.; Ranke, W.; Weiss, W. Water on FeO(111) and Fe3O4(111): Adsorption Behavior on Different Surface Terminations. J. Phys. Chem. B 2000, 104, 3224–3236. (12) Goniakowski, J.; Noguera, C. Theoretical Investigation of Acid-Base Properties of Oxide Surfaces. J. Adhes. 1996, 57, 91–114. (13) Hu, X. L.; Carrasco, J.; Klimeš, J.; Michaelides, A. Trends in Water Monomer Adsorption and Dissociation on Flat Insulating Surfaces. Phys. Chem. Chem. Phys. 2011, 13, 12447–12453. (14) Fester, J.; García-Melchor, M.; Walton, S.; Bajdich, M.; Li, Z.; Lammich, L.; Vojvodic, A.; Lauritsen, J. V. Edge Reactivity and Water-Assisted Dissociation on Cobalt Oxide Nanoislands. Nat. Commun. 2017, 8, 14169. (15) Shin, H. J.; Jung, J.; Motobayashi, K.; Yanagisawa, S.; Morikawa, Y.; Kim, Y.; Kawai, M. StateSelective Dissociation of a Single Water Molecule on an Ultrathin MgO Film. Nat. Mater. 2010, 9, 442–447. (16) Paracchino, A.; Laporte, V.; Sivula, K.; Grätzel, M.; Thimsen, E. Highly Active Oxide Photocathode for Photoelectrochemical Water Reduction. Nat. Mater. 2011, 10, 456–461. (17) Kwon, Y.; Soon, A.; Han, H.; Lee, H. Shape Effects of Cuprous Oxide Particles on Stability in Water and Photocatalytic Water Splitting. J. Mater. Chem. A 2015, 3, 156–162. (18) Riplinger, C.; Carter, E. A. Cooperative Effects in Water Binding to Cuprous Oxide Surfaces. J. Phys. Chem. C 2015, 119, 9311–9323. (19) Yu, X.; Zhang, X.; Wang, S.; Feng, G. A Computational Study on Water Adsorption on Cu2O(111) Surfaces: The Effects of Coverage and Oxygen Defect. Appl. Surf. Sci. 2015, 343, 33–40. (20) Li, Y.; Yan, L.; Wang, G. Adsorption and Dissociation of H2O on Cu2O(100): A Computational Study. J. Nat. Gas Chem. 2011, 20, 155–161. (21) Cox, D. F.; Schulz, K. H. H2O Adsorption on Cu2O(100). Surf. Sci. 1991, 256, 67–76. 16 ACS Paragon Plus Environment

Page 16 of 23

Page 17 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(22) Kronawitter, C. X.; Riplinger, C.; He, X.; Zahl, P.; Carter, E. A.; Sutter, P.; Koel, B. E. HydrogenBonded Cyclic Water Clusters Nucleated on an Oxide Surface. J. Am. Chem. Soc. 2014, 136, 13283–13288. (23) Jensen, F.; Besenbacher, F.; Lægsgaard, E.; Stensgaard, I. Oxidation of Cu(111): Two New Oxygen Induced Reconstructions. Surf. Sci. Lett. 1991, 259, L774–L780. (24) Sträter, H.; Fedderwitz, H.; Groß, B.; Nilius, N. Growth and Surface Properties of Cuprous Oxide Films on Au(111). J. Phys. Chem. C 2015, 119, 5975–5981. (25) Nilius, N.; Fedderwitz, H.; Groß, B.; Goniakowski, J.; Noguera, C. Incorrect DFT- GGA Predictions of the Stability of Non-Stoichiometric/Polar Dielectric Surfaces: The Case of Cu2O(111). Phys. Chem. Chem. Phys. 2016, 18, 6729 – 6733. (26) Henderson, M. a. An HREELS and TPD Study of Water on TiO2(110): The Extent of Molecular versus Dissociative Adsorption. Surf. Sci. 1996, 355, 151–166. (27) Hodgson, a.; Haq, S. Water Adsorption and the Wetting of Metal Surfaces. Surf. Sci. Rep. 2009, 64, 381–451. (28) Michaelides, A.; Morgenstern, K. Ice Nanoclusters at Hydrophobic Metal Surfaces. Nat. Mater. 2007, 6, 597–601. (29) Soon, A.; Todorova, M.; Delley, B.; Stampfl, C. Thermodynamic Stability and Structure of Copper Oxide Surfaces: A First-Principles Investigation. Phys. Rev. B 2007, 75, 125420. (30) 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, 8, 585–589. (31) Tatarkhanov, M.; Ogletree, D. F.; Rose, F.; Mitsui, T.; Fomin, E.; Maier, S.; Rose, M.; Cerda, J. I. Metal- and Hydrogen-Bonding Competition during Water Adsorption on Pd (111) and Ru (0001). 2009, 111, 18425–18434.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

TOC IMAGE


Page 18 of 23

Page 19 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


215x279mm (220 x 220 DPI)

ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure_2 102x102mm (300 x 300 DPI)


Page 20 of 23

Page 21 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure_3 102x89mm (300 x 300 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure_4 142x92mm (300 x 300 DPI)


Page 22 of 23

Page 23 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


TOC graphic 79x47mm (300 x 300 DPI)


Surfaces: A Scanning Tunneling Microscopy Study - American

Recommend Documents