Interaction of Atomic Hydrogen with the Cu2O(100) and (111) Surfaces

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C: Surfaces, Interfaces, Porous Materials, and Catalysis 2

Interaction of Atomic Hydrogen with the CuO(100) and (111) Surfaces Heloise Tissot, Chunlei Wang, Joakim Halldin Stenlid, Mohammad Panahi, Sarp Kaya, Markus Soldemo, Milad Ghadami Yazdi, Tore Brinck, and Jonas Weissenrieder J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b03888 • Publication Date (Web): 16 Aug 2019 Downloaded from pubs.acs.org on August 23, 2019

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The Journal of Physical Chemistry

Interaction of Atomic Hydrogen with the Cu2O(100) and (111) Surfaces Heloise Tissot1, Chunlei Wang1, Joakim Halldin Stenlid2, Mohammad Panahi4,5, Sarp Kaya4,5, Markus Soldemo1, Milad Ghadami Yazdi1, Tore Brinck3, Jonas Weissenrieder1,* 1

Material Physics, School of Engineering Sciences, KTH Royal Institute of Technology, SE-100 44 Stockholm, Sweden 2

Department of Physics, Albanova University Center, Stockholm University, SE-106 91, Stockholm, Sweden 3

Applied Physical Chemistry, Department of Chemistry, KTH Royal Institute of Technology, SE-100 44 Stockholm, Sweden 4

Koç University TUPRAS Energy Center, Istanbul, Turkey

5

Chemistry Department, Koç University, Istanbul, Turkey

ABSTRACT Reduction of Cu2O by hydrogen is a common preparation step for heterogeneous catalysts, however, a detailed understanding of the atomic reaction pathways is still lacking. Here we investigate the interaction of atomic hydrogen with the Cu2O(100):(3,0;1,1) and the Cu2O(111): (3 × 3)R30° surfaces using scanning tunneling microscopy (STM), low energy electron diffraction, temperature programmed desorption (TPD), and X-ray photoelectron spectroscopy (XPS). The experimental results are compared to density functional theory simulations. At 300 K we identify the most favorable adsorption site on the Cu2O(100) surface, hydrogen atoms bind to an oxygen site located at the base of the atomic rows intrinsic to the (3,0;1,1) surface. The resulting hydroxyl group subsequently migrate to a nearby Cu trimer site. TPD analysis identifies H2 as the principal desorption product. These observations imply that H2 is formed through a disproportionation reaction of surface hydroxyl groups. The interaction of H with the (111) surface is more complex including coordination to both Cu+ and Ocus sites. STM and XPS analysis reveal the formation of metallic copper clusters on the Cu2O surfaces after cycles of hydrogen exposure and annealing. The interaction of the Cu clusters with the substrate is notably different for the two surface terminations studied: after annealing the Cu clusters coalesce on the (100) termination and the (3,0;1,1) reconstruction is partially recovered. Clusters formed on the (111) surface are less

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prone to coalescence and the (3 × 3)R30° reconstruction was not recovered by heat treatment, indicating a weaker Cu cluster to support interaction on the (100) surface. * corresponding author: [email protected]

INTRODUCTION The chemical interaction of hydrogen with metal oxide surfaces plays an important role in several heterogeneous, oxide-catalyzed reactions through activation of the catalysts by reduction into suboxides or metal.1,2 Cu2O surfaces are of particular interest to catalytic3,4 and photo-catalytic water splitting5–8 reactions. However, despite its promising properties, commercial application of Cu2O as catalyst for water decomposition under visible light irradiation6 has not been realized since Cu2O based electrodes are susceptible to rapid photo-degradation in aqueous solution. The catalyst deactivation presumable proceeds via reduction of the catalytically active Cu+ sites.7,9 Despite Cu2Os potential importance, H2 adsorption and desorption, as well as substrate reduction processes taking place on Cu oxide surfaces are poorly understood at a molecular level. The adsorption of hydrogen on the reconstructed Cu2O(100) surface has previously been studied in a seminal paper by Schulz et al.10 Under ultrahigh vacuum conditions, H2 does not dissociatively adsorb on the Cu2O(100) surface. Temperature programmed desorption (TPD) experiments found no evidence for hydrogen uptake by the oxide, neither at 110 K nor 300 K, for doses as high as 104 Langmuir and pressures up to 1 × 10-6 mbar, suggesting that dissociation is hindered by a high barrier. To facilitate favorable conditions for hydrogen adsorption on the surface, Schulz et al.10 dissociated H2 using a hot Pt filament placed in proximity of the sample surface. TPD analysis following the adsorption of atomic hydrogen on the surface revealed mainly H2 desorption and only minor fractions of water as a desorption product. The large excess of H2 desorption compared to water was interpreted as an interaction of a majority of the adsorbed hydrogen atoms with copper cations to form Cu-H bonds. Surprisingly, X-ray photoelectron spectroscopy (XPS) analysis did not detect reduction of the copper cations after the thermal desorption, not even for high coverages.10 The reduction of Cu2O by H2 is however well-known to take place at ambient H2 pressures, both for Cu2O oxides surfaces11 and powders.12 X-ray diffraction analysis of Cu2O powder under a flow of H2 /He (5% H2 balanced with He, at a total pressure of 1 atm) shows oxide 2 ACS Paragon Plus Environment

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reduction and formation of metallic Cu commence near 575 K. Extended annealing results in a complete reduction of the catalyst.12 This contradiction in results raises questions whether the conditions for atomic H adsorption in UHV are relevant for modelling reduction steps in applied catalysis. To address the above questions, we study the interaction of atomic hydrogen with the Cu2O(100):(3,0;1,1) and the Cu2O(111): (3 × 3)R30° surfaces at room temperature and their possible reduction by means of several techniques: scanning tunneling microscopy (STM), low energy electron diffraction (LEED), TPD, XPS, and simulations within the framework of density functional theory (DFT). Low doses of hydrogen were first investigated to provide an understanding of the process for the initial atomic hydrogen adsorption and the tentative formation of hydroxyl groups on the surface. Higher doses of atomic hydrogen and cycles of temperature treatments were then performed in order to study the reduction processes of the surfaces.

EXPERIMENTAL DETAILS The Cu2O(100) and Cu2O(111) single crystals used in this study are manufactured from a natural crystal acquired from the Surface Preparation Laboratory, the Netherlands. The cleaning procedure of the surfaces includes cycles of argon ion sputtering (0.5 kV, 20 min) followed by annealing in ultrahigh vacuum (at 900 K) or in oxygen gas (at 750 K, P(O2) = 3 × 10-6 mbar). For the LEED and STM studies, the crystal cleanness and order were first confirmed by the sharpness of the diffraction spots in the LEED pattern. The STM study was carried out using an Omicron VT-STM operated in constant current imaging mode with electrochemically etched tungsten tips. The STMchamber is attached to a preparation chamber equipped with ion sputter gun, high-precision leakvalves for gases, sample annealing, and LEED-apparatus. The presented LEED patterns were obtained using this LEED. The base pressures in both preparation chamber and STM chamber are in the range of low 10−10 mbar. Exposure of clean Cu2O(100) and (111) surfaces to atomic hydrogen at room temperature was performed by cracking H2 using an Omicron atomic hydrogen source (P(H2) = 2 × 10-8 mbar, using an emission current of 30 - 40 mA). The sample was placed ~ 20 cm from the nozzle of the hydrogen source.



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The XPS analysis was carried out at the UHV-XPS endstation at the beamline I311 on the MAX II storage ring of the National Swedish Synchrotron Facility MAX IV Laboratory in Lund, Sweden13 and at the PEARL beamline of the Swiss Light Source, Paul Scherrer Institute, Villigen, Switzerland.14 At I311, the analysis chamber was equipped with a Scienta SES200 electron analyzer. In direct connection to the analysis chamber was a preparation chamber equipped with facilities for argon ion sputtering, sample annealing, high precision leak valves, and a LEED apparatus. The XPS analysis at the PEARL beamline was conducted using a Scienta EW 4000 electron analyzer. Sample preparation was conducted in a dedicated preparation chamber equipped with facilities for argon ion sputtering, sample annealing, high precision leak valves, and a LEED apparatus.14 All XPS binding energies were referenced to the Fermi level measured at a tantalum foil in direct electrical contact with the sample. H2 and CO TPD measurements were conducted using a quadruple mass spectrometer (Hiden HAL 201). The spectrometer is mounted in a differentially pumped gold coated shield with a 5 mm diameter aperture, located B > C > D, whereas Ī(r), that also account for the local charge-transfer ability, ranks the sites as A = B > C > D (see Figure S8 in the supporting information). This in combination with our finding that the 16 ACS Paragon Plus Environment

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B-site displays the most favorable interaction with H suggests that the H adsorption on the Cu2O surface is more likely controlled by charge-transfer than by electrostatics, but also that additional effects such as structural relaxation is important for determining the local surface basicity of the Cu2O(100) surface. Involvement of site B in the H bonding is, in addition, in line with the experimental observation: as mentioned earlier the bright protrusions associated to OH groups on the STM images (Figure 1) are in between the atomic rows of the surface. However, the small difference in the adsorption energies between the surface sites might suggest adsorption on multiple sites and explain the loss of the surface reconstruction observed by LEED and STM as well as the disappearance of surface state peaks in the O 1s spectra for high atomic hydrogen coverage. Another possible mode of H atom adsorption is via expulsion of a surface oxygen atom and migration to another adsorption site. The most favorable adsorption mode in this mechanism is migration of an OH from the B site to the Cu-trimer D site. These modes are energetically equivalent and separated by a small diffusion barrier of 0.55 eV as determined by transition state calculations employing the climbing image nudge elastic band method.37 As a matter of fact, calculated O 1s core level shifts (CLS) for OH at the different adsorption sites suggest that the OHad at the D site is the most likely to form among the considered surface species. This species has a CLS of 0.95 eV, which is close to the experimental value of 1.1 eV. The corresponding values for the A, B and C sites are 1.8, 1.6 and 1.8 eV, respectively. Formation of H2O from the OHad and an H atom is energetically beneficial (ΔE=-0.28 eV on the (3,0;1,1) surface) and it prefers binding to the Cu-trimer site. However, it is known from studies of H2O interactions on the (100) surface that there is a substantial barrier for the H2O dissociation/association reaction.26 H2O is also likely to desorb quickly at elevated temperatures. The CLS of 2.16 for H2O is in line with the experimental shift of 2.2 eV. We can also note that both the OH and H2O shifts are in agreement with experimental26 and computed (H2O = 1.87 eV, OH = 0.81 eV, see details in the Supporting Information) O 1s peak positions for H2O and OH in a mixed H2O-OH monolayer on the unreconstructed Cu-terminated Cu2O(100) surface.



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Table 1. Hydroxyl groups positions, and electronic (ΔEad) and Gibbs free energies (ΔGad) in eV (Red balls represent oxygen atoms and brown copper atoms).

a

Site

Location

ΔEada ΔGada SCLSb

A

On ridge

-0.26

0.18

1.86

B

Valley (cell center)

-0.54

-0.10

1.61

C

Base of ridge

-0.41

-0.04

1.87

D

Copper trimer

-0.23

0.17

n.a.c

D’

OH migrated from B

-0.54

-0.09

0.95

Adsorption energies for the reaction ½H2(g)  Had. b O 1s surface core level shift (SCLS) in eV for O in OHad as

computed by DFT using the procedure in ref 23. c H adsorbed directly on Cu.

The case of the (111) surface is more difficult to interpret, after atomic hydrogen adsorption, the STM images show a disturbance of the hexagonal arrangement of the bright protrusions attributed to oxygen vacancies (Figure 6). The results indicate that hydroxyls groups may be formed by interaction with surface oxygen atoms in between the oxygen vacancies. On the Cu2O(111), the (√3 × √3)R30° reconstruction is attributed to the removal of one-third of the undercoordinated surface oxygen anions (OCUS), the bright protrusions being centered on these vacancies, meaning that hydrogen atoms are binding to the remaining (OCUS) atoms located in between these oxygen vacancies. Our DFT calculations indeed show that H adsorption at (OCUS) is likely at the levels of exposures in this study. However, in agreement with previous computational studies,42 we find that H first occupies surface exposed copper sites on the (√3 × √3)R30° reconstructed surface as presented in Table 2.


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Table 2. Adsorption positions and adsorption energies (ΔEad) relative the most favored site in eV on the (3 × 3)R30° reconstructed Cu2O(111) surface. (Purple balls represent lattice oxygen atoms and dark red OCUS atoms. Brown balls are copper lattice atoms whereas dark brown depicts CuCUS).

If undercoordinated copper (CuCUS) atoms are present, these bind H 0.2 eV stronger than OCUS, and if the surface lacks CuCUS the H first adsorbs at the Cu exposed under the oxygen vacancies (see supporting information Table S1). It is interesting to note that for the Cu2O(100) surface the amount of OH groups observed in the XPS spectra after water and atomic hydrogen adsorption is the same. Even though XPS shows similar degree of hydroxyl formation after atomic H and H2O adsorption, the molecular adsorption mechanisms on the surface are obviously different and the resulting surface structures also differ (cf. (1 × 1)/c(2 × 2) in the case of H2O adsorption and dissociation,26 and (3,0;1,1) in the case of H). In the case of atomic hydrogen, the formation of an OH group will necessarily involve a surface oxygen atom while water dissociation introduces an additional oxygen atom to the system. Nevertheless, the similar coverage obtained for both atomic hydrogen and water adsorption suggests that this value correspond to the maximum coverage that can be reached in UHV due to structural issues and steric effects. Cycles of extended exposure to atomic hydrogen and temperature treatments were performed in order to study the surface reduction process. The results show the formation of metallic copper clusters on the surface after dosage of atomic hydrogen and annealing at moderate temperature. Annealing in UHV facilitates the extraction of a surface oxygen atom to produce water and a reduced surface site. These findings confirm the observations made on Cu2O electrodes used for water splitting reactions, for which deactivation was observed due to its reduction into metallic 19 ACS Paragon Plus Environment


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copper in the presence of H2 produced by the reaction.6,9 Furthermore, it is interesting to compare the (100) and (111) surfaces, the photo-degradation of electrode surfaces originates in the deactivation of Cu+ centers, but LEED and STM images show that after annealing the Cu2O(100) surface, the (3,0;1,1) reconstruction is partially recovered as well as the surface Cu+ active sites – although the available reconstructed surface area is smaller due to the presence of copper clusters. On the other hand, the Cu2O(111) does not recover the (3 × 3)R30° surface reconstruction, and the surface remains rough and is probably covered by metallic copper. CONCLUSIONS To obtain a molecular level understanding of the interaction of atomic hydrogen with the Cu2O(100):(3,0;1,1) and the Cu2O(111):(3 × 3)R30° surfaces and to determine possible reduction pathways, we employed several surface science techniques such as STM, LEED, TPD, and high resolution XPS. The results were compared to DFT simulations. Low dose experiments combined with DFT results allow us to identify the most favorable adsorption site of atomic hydrogen on the Cu2O(100) surface as the surface oxygen atom, located at the base of the atomic rows of the reconstructed surface (labeled B). The resulting hydroxyl group subsequently migrate to a nearby Cu trimer site (site D). TPD analysis identifies H2 as the principal desorption product. These observations imply that H2 is formed through a disproportionation reaction of surface hydroxyl groups. The interaction of H with the (111) surface is more complex including coordination to both Cu+ and Ocus sites. High doses result in surfaces decorated by clusters on both the (100) and the (111) surfaces. Finally, in contrast to what was reported by Schultz and Cox10, STM and XPS results show the formation of metallic copper clusters after thermal treatments of the atomic hydrogen exposed surfaces. This finding is also in agreement with observations from hydrogen treatment of Cu2O powders.12 The support interaction of the metallic copper cluster is different for the two surfaces: after annealing, the (3,0;1,1) reconstruction of the (100) surface can be partially recovered while the (3 × 3)R30° reconstruction on the (111) surface the is not recovered by the same procedure. These observations underline the importance in considering the atomic structure of the surface when investigating reductive deactivation mechanisms for Cu2O electrodes.

ACKNOWLEDGEMENT 20 ACS Paragon Plus Environment

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This work was funded by the Swedish Research Council (VR), the Knut och Alice Wallenbergs stiftelse, the Ragnar Holm foundation for Heloise Tissot’s fellowship, and the Trygger’s foundation for Chunlei Wang’s fellowship. S.K. and M.P thank TARLA for collaborative research afford. JHS acknowledge the Åforsk foundation for financial support. We thank Matthias Muntwiler (PEARL, SLS) for excellent support during the beamtime. The MAX IV staff is gratefully acknowledged for their generous support during the beamtime. The Swedish National Infrastructure for Computing (SNIC) is acknowledged for providing computational resources at the National Supercomputer Centre in Linköping University NSC as well as at the PDC Centre for High Performance Computing (PDC-HPC).

SUPPORTING INFORMATION DESCRIPTION Supporting information contains STS analysis performed on the Cu2O(100):(3,0;1,1) surface after exposure to atomic hydrogen, XPS data after water adsorption on the same surface, additional XPS analysis of Cu2O(100) and Cu2O(111) surfaces after exposure to atomic hydrogen, and CO TPD analysis of clean and reduced and Cu2O(100). Also including DFT H adsorption energies for CuCUS vacant Cu2O(111), computed core level shifts for H2O and OH on Cu2O(100), simulated STM images and density of states (DOS) for the favored Had structure at low coverage, as well as local basicity analysis of the Cu2O(100) surface using two local properties computed by DFT: the electrostatic potential and the local average ionization energy.

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Interaction of Atomic Hydrogen with the Cu2O(100) and (111) Surfaces

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