Formaldehyde Adsorption on the Anatase TiO2(101) Surface

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Formaldehyde Adsorption on the Anatase TiO (101) Surface - Experimental and Theoretical Investigation Martin Setvin, Jan Hulva, Honghong Wang, Thomas Simschitz, Michael Schmid, Gareth S. Parkinson, Cristiana Di Valentin, Annabella Selloni, and Ulrike Diebold J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b01434 • Publication Date (Web): 10 Apr 2017 Downloaded from http://pubs.acs.org on April 12, 2017

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The Journal of Physical Chemistry

Formaldehyde Adsorption on the Anatase TiO2 (101) Surface - Experimental and Theoretical Investigation Martin Setvin,1,* Jan Hulva,1 Honghong Wang,2 Thomas Simschitz,1 Michael Schmid,1 Gareth S. Parkinson,1 Cristiana Di Valentin,3 Annabella Selloni,2 Ulrike Diebold1

1

Institute of Applied Physics, TU Wien, Wiedner Hauptstrasse 8-10/134, 1040 Vienna,

Austria 2

Department of Chemistry, Princeton University, Frick Laboratory, Princeton, New Jersey

08544, United States 3

Dipartimento dei Scienza dei Materiali, Università di Milano-Bicocca, Via Cozzi 55, 20125

Milano, Italy

*email: [email protected]

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ABSTRACT: Formaldehyde (CH2O) adsorption on the anatase TiO2 (101) surface was studied with a combination of experimental and theoretical methods. Scanning tunneling microscopy (STM), non-contact atomic force microscopy (nc-AFM), temperature programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS) were employed on the experimental side. Density Functional Theory (DFT) was used to calculate formaldehyde adsorption configurations and energy barriers for transitions between them. At low coverages (< 0.25 monolayer) CH2O binds via its oxygen atom to the surface 5coordinated Ti atoms Ti5c (monodentate configuration). At higher coverages, many adsorption configurations with comparable adsorption energies coexist, including a bidentate configuration and paraformaldehyde chains. The adsorption energies of all possible adsorption configurations lie in the range from 0.6 to 0.8 eV. Upon annealing, all formaldehyde molecules desorb below room temperature; no other reaction products were detected.


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INTRODUCTION TiO2 is a prototypical photocatalytic material,1-2 used for decomposing organic pollutants in water treatment applications. It is also interesting for solar-light harvesting applications such as photocatalytic water splitting or Graetzel solar cells.3 TiO2 crystallizes in two main forms, rutile and anatase,4 where the latter is more frequently used in applications. Formaldehyde (H2CO) is an important chemical compound used for a wide variety of purposes. Industrially it is mainly produced by oxidation of methanol on metal catalysts, 5-6 while in photocatalysis it is an intermediate in several reduction and oxidation processes.2, 7-13 In contrast to the many detailed investigations of simple molecules on TiO2,14-15 studies of formaldehyde are scarce, possibly due to its high reactivity and, consequently, complex adsorption scheme. Recent studies performed on the rutile (110) surface16-20 indicate several formaldehyde adsorption configurations: First, in a monodentate configuration it can bind weakly via its oxygen atom to the undercoordinated surface Ti5c atom. Alternatively, it can adsorb in a bidentate fashion, also called “dioxomethylene” configuration, with its O atom bound to a surface Ti5c atom, and its C atom to an adjacent surface O2c (bridging oxygen) atom. Some reports have also mentioned the possibility of forming paraformaldehyde chains at the rutile surface.16 Last, formaldehyde can directly react with defects such as surface oxygen vacancies on the rutile (110) surface.18-19 TPD measurements on rutile TiO2 (110) revealed that thermal annealing of adsorbed formaldehyde may result in formation of further products, for example ethene.17-19 The aim of this work is to obtain a detailed understanding of formaldehyde adsorption on the anatase TiO2(101) surface, using a combined experimental and computational approach. First we introduce temperature-programmed desorption (TPD) results, which allow us to determine the adsorption modes and adsorption energies in the coverage regime from submonolayer up to multilayers. Next we present XPS results of the C1s and O1s peaks, 3 ACS Paragon Plus Environment


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which show coverage-dependent chemical changes in both the O and C atoms of the formaldehyde molecules. DFT calculations are used to gain insight into the adsorption configurations and energetics of a single CH2O molecule on the stoichiometric anatase (101) surface. The calculations reveal two dominant adsorption configurations: a monodentate structure with an adsorption energy of 0.62 eV and a bidentate one with an adsorption energy of 0.69 eV. Transition from the monodentate to the bidentate structure requires the breaking of the C=O double bond and the rehybridization of the carbon atom from sp 2 to sp3. It thus involves an energy barrier that we determined by nudged elastic band (NEB) calculations. 21 We also obtain insights into the interactions between adsorbed molecules by calculating the adsorption energies of selected configurations of two, three and four CH 2O molecules in a repeating cell. For direct experimental information about the adsorption configurations, we employed a combination of scanning tunneling microscopy (STM) and non-contact atomic force microscopy (nc-AFM). Single adsorbed formaldehyde molecules can be imaged by STM at low coverages (~0.1 monolayer, ML) but the imaging becomes very unstable at higher coverages. This problem arises from electron tunneling through the layer and can be overcome by using nc-AFM with zero sample bias.

MATERIALS AND METHODS The TPD and XPS measurements were carried out in a UHV system with a base pressure of 5 × 10-11 mbar. The anatase sample was mounted on a Ta back plate, cooled by a Janis ST-400 UHV liquid-He flow cryostat, and heated by direct current through the back plate. The temperature was measured by a K-type thermocouple spot-welded to the sample plate, and calibrated using multilayer desorption of several gases (O2, H2O, CO). The resulting uncertainty in the absolute temperature reading is ±3 K. During the TPD 4 ACS Paragon Plus Environment

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measurements, the sample was biased at −100 V to prevent electrons from the quadrupole’s filament reaching the sample surface. Gases were dosed by an effusive molecular beam with a nearly perfect top-hat-shape profile and accurately known incident flux.22-23 This produces a beam spot of ≈3.5 mm diameter at the sample (sample size 4 × 6 mm2). Thus, only the TiO2(101) surface is exposed to the molecular beam. The dose rate under these conditions is approximately 1.2 ML/minute. For the TPD measurements the formaldehyde was dosed at T=51 K and a linear temperature ramp of 1 K/s was used. A HIDEN quadrupole mass spectrometer in a line-of-sight configuration was used for detection of the desorbed species. The amount of dosed gases was calibrated according to the method described in ref.

23

. One

monolayer (ML) is defined as 1 molecule per surface 5-coordinated titanium atom (Ti5c) and corresponds to an exposure of 1.6 Langmuir (L); this already includes a correction for the measured value of the sticking coefficient S=0.85. XPS spectra were measured with a hemispherical electrostatic energy analyzer (SPECS Phoibos 150), using a monochromatized Al Kα X-ray source (SPECS Focus 500). Data were recorded at a sample temperature of T = 51 K. XPS was measured under an exit angle 60° off normal. STM and nc-AFM measurements were performed at T = 4.8 K in a UHV chamber with a base pressure below 2 × 10-11 mbar, equipped with a commercial Omicron q-Plus LT head. Controlled low-temperature annealing of the sample was performed in a manipulator cooled by flowing nitrogen gas. The temperature was measured by a K-type thermocouple attached to the sample holder, resulting in a temperature uncertainty of ± 10 K. Tuning-forkbased AFM sensors with a separate wire for the tunneling current24-25 were used (k = 3750 N/m, f0 = 47500 Hz, Q ~ 20000). Electrochemically etched W tips were glued to the tuning fork and cleaned in-situ by field emission and self-sputtering in 1 × 10-4 Pa Argon.26 Before approaching the anatase surface, the tip was purposely functionalized by a CO molecule,27 although we cannot exclude picking up a formaldehyde molecule (or constituents thereof)



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while imaging. We used a single-crystalline mineral anatase TiO2(101) sample, naturally doped by 1% Nb.28 For all experiments the surface was prepared by ex-situ cleaving or polishing (both types were used) and subsequent cleaning in vacuum by cycles of Ar+ sputtering (1 keV) and annealing to 950 K.29 For STM and nc-AFM measurements the formaldehyde was dosed via a leak valve; the formaldehyde gas was prepared by thermal decomposition of paraformaldehyde powder at 360 K. DFT calculations were performed within the plane-wave pseudopotential scheme as implemented in the Quantum ESPRESSO package.30 We used the generalized gradient approximation of Perdew, Burke and Ernzerhof (PBE)31 with the addition of on-site Hubbard U repulsion32 on the Ti 3d orbitals. We took U = 3.9 eV, as obtained from cRPA calculations29. Electron–ion interactions were described using ultrasoft pseudopotentials.33 We expanded the electronic states in plane waves using a kinetic energy cut-off of 25.0 (200.0) Ry for the smooth part of the wave-function (augmented charge density). The anatase (101) surface was modeled using a repeated slab geometry with lattice parameters optimized at the PBE level and a vacuum width of 18 Å between adjacent slabs. We considered a slab of 3 TiO2 bi-layers, with a rectangular surface supercell of dimensions (10.26 × 11.31 Å2). Only the -point was used to sample the Brillouin zone. Structural optimizations were performed relaxing all atomic positions until residual forces were smaller than 0.05 eV/Å. Energy barriers between different adsorption structures were estimated using the nudged-elastic band (NEB) method21 and pathways were relaxed until forces converged below 0.05 eV/Å. Constant-density (10-5 electrons per bohr3) STM images were calculated by integrating the local density of states in an energy window of ~2.5 eV from the conduction band minimum.

RESULTS AND DISCUSSION


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Temperature Programmed Desorption

\ Figure 1. TPD spectra of formaldehyde desorption from the anatase (101) surface (m/z=29 is shown). a) Coverage range 0 to 1 ML, b) 1 to 2 ML. The results of our TPD measurements are reported in Figure 1, where panels (a) and (b) show TPD spectra in the coverage regime of 0 to 1 ML, and 1 to 2 ML, respectively. We note that in all cases the amount of desorbed formaldehyde corresponded to the dosed amount, and no other reaction product was detected. The formaldehyde desorption exhibits a peculiar behavior. At very low coverages, i.e., in the curves for 0.06 and 0.12 ML, we observe one peak with a maximum at 245 and 240 K, respectively. These peaks have their high-temperature edges aligned, which can be either interpreted as a second-order desorption, or first-order desorption where the adsorption energy decreases with the coverage.34 The desorption regime changes at increasing coverages (0.25 to 1 ML); the peak maximum shifts



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to increasingly higher temperatures, up to 257 K. Here the low-temperature slopes are aligned, like for zero-order desorption. In our case this is due to desorption from islands;23, 35 we will show below that formaldehyde can form structures with slight attractive interaction. The TPD results were analyzed by the inversion analysis method according to ref. 34. The resulting adsorption energy is Eads = (0.7±0.1) eV and the frequency prefactor v0 = 10(14±1) s-1. This frequency prefactor is comparable to values reported for desorption of organic molecules of similar sizes.36-37 For coverage above 1 ML (Figure 1b) we observe the multilayer peak at 100 K, as expected, as well as a modification of the first-monolayer desorption. The original peak at ~250 K becomes smaller and a shoulder appears at the higher-temperature side (282 K). While the total area of the peak plus shoulder stays constant, the high-temperature shoulder can be attributed to a separate adsorption configuration (first order) with an adsorption energy of (0.8±0.1) eV. The cracking pattern of the spectra confirm that the desorbing species are formaldehyde; again no other reaction products were detected. The TPD of formaldehyde from the anatase (101) surface is considerably different from a previous report for the rutile (110) surface.16 There, the desorption peaks extend above room temperature and other reaction products are formed, for example ethylene.17, 38 In recent STM studies on rutile (110), surface oxygen vacancies were identified as the reactive sites for ethylene production.18-19 We attribute the lack of other reaction products on anatase (101) to the fact that this surface does not contain oxygen vacancies.39 X-ray Photoelectron Spectroscopy.


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Figure 2. XPS spectra of the principal core-level peaks after adsorbing different amounts of formaldehyde. a) C1s, b) O1s, c) Ti2p. The inset in b) shows a detail of the region between EB = 532 and 536 eV, which corresponds to the signal from adsorbed formaldehyde. Spectra were measured on a layer dosed at 51 K (thin lines), and after annealing to 140 K (thick lines). XPS measurements allowed us to gain further insight into the chemical changes in the adsorbed molecules. Figure 2 shows XPS spectra of the C1s, O1s, and Ti2p levels after dosing different amounts of formaldehyde. The spectra were measured just after dosing the molecules at T = 51 K, and also after annealing the layer to T = 140 K (the sample was cooled back to 51 K for the XPS measurements). The annealing step removes any adsorbate above 1 ML coverage, and triggers processes that require an activation energy. At 0.25 ML coverage, the spectra before and after annealing are essentially identical. The C1s peak is located at 9 ACS Paragon Plus Environment


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288.9 eV, comparable to the value reported on the rutile (110) surface.17 The O1s signal from formaldehyde is found at ~534.5 eV (see the inset in Figure 2b; the peak at 531.2 eV originates from the lattice oxygen of TiO2). Dosing higher amounts of formaldehyde at T = 51 K leads to an increase of the C1s and O1s peak heights, while the peak positions do not change significantly compared to 0.25 ML. After annealing to 140 K, however, both peaks shift towards lower binding energies: the C1s state shifts to 288.3 eV and the O1s forms a wide shoulder centered at ~533.5 eV. Spectra recorded for a coverage of 1.6 ML (black curve) follow the trends observed at 1.0 ML. We note that here the annealing results in desorption of any formaldehyde molecules above the first monolayer. No changes were detected in the shape and position of the Ti2p peak (Figure 2c). This indicates that the formaldehyde is not deprotonated during the adsorption; as excess H usually causes a Ti3+ shoulder at the lower BE side.4 Further, no significant band bending is observed; indicating little, if any charge transfer. The TiO2 substrate is neither reduced nor oxidized by the formaldehyde. The XPS data clearly show that the adsorbed formaldehyde undergoes chemical changes at higher coverages, although the data allow several possible interpretations. The formaldehyde O1s peak shifts towards lower binding energies, which may indicate weakening or breaking the C=O double bond and formation of another bond with a coadsorbed molecule or with the surface. This can be an indicative of changing the adsorption configuration from monodentate to bidentate, or formation of paraformaldehyde at the surface, or hydrogen bond donation to the formaldehyde oxygen atom.40 The slight chemical shift in the carbon peak is consistent with all these interpretations. Its shifted position at 288.3 eV is consistent with the O-C-O environment (bidentate adsorption or paraformaldehyde), as well as with the hydrogen bonding picture.40 We used DFT calculations to gain insight into the energetics of these configurations.


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DFT Calculations. First we investigated the energetics of a single CH2O molecule adsorbed in a (4 × 1) surface unit cell; our results are shown in Figure 3. The three configurations are based on a previous theoretical work.41 In the first two configurations, denoted as I.a and I.b, the formaldehyde is bound to a surface Ti5c atom via its oxygen atom. In addition, the molecule forms a hydrogen bond with the surface O2c atom either in the same Ti5c-O2c row (configuration I.a, Eads = 0.58 eV), or across the row (configuration I.b, Eads = 0.62 eV). The configuration II is significantly different. The molecule has the strongest bonding to the substrate (Eads = 0.69 eV), as the formaldehyde is bound via both its O and C atoms to the substrate Ti5c and O2c atoms, respectively. While the configurations I.a and I.b keep the original chemical state of the molecule, configuration II requires breaking the C=O double bond and a change of the carbon hybridization from sp2 to sp3. Therefore, even though energetically favored, this configuration can be accessed only after overcoming a kinetic barrier. The energy barriers separating the three calculated adsorption configurations were determined by NEB calculations, see lower panel of Figure 3. The energy barrier between configurations I.a and I.b is small, 0.16 eV. On the other hand, the transition from I.a to II has an energy barrier of 0.48 eV, so that the barrier between I.b and II is 0.52 eV. The highest point (transition state) along the pathway is already close to the desorption energy, suggesting that there might be a subtle competition between desorption and transition into the state II.



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Figure 3. Calculated adsorption configurations of an isolated formaldehyde molecule on the anatase (101) surface. Left: structural models (O red, Ti silver, and C darker gray). Right: calculated STM images. (Here “C” marks the position of the carbon atom). The calculated adsorption energies are 0.58 eV for the configuration I.a, 0.62 eV for I.b, and 0.69 eV for configuration II. Selected bond lengths (in Å) are shown in the structural models. The plot in the lower panel shows the minimum-energy pathways and barriers for transitions between the different adsorption configurations, as obtained from NEB calculations. 12 ACS Paragon Plus Environment

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Figure 4. Calculated adsorption configurations of two (a,b,c), three (d) and four (e) formaldehyde molecules on anatase (101). The adsorption energies are shown below each configuration (average adsorption energy per molecule). Motivated by the peculiar TPD and XPS results reported above, we also investigated the adsorption of CH2O pairs. Molecules adsorbed in configuration Ib. show a slight 13 ACS Paragon Plus Environment


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repulsion when they approach each other, see Figures 4a,b. The calculations show an effective repulsion of 10 and 60 meV for molecules adjacent along the [1̅11] and [010] directions, respectively. This is in agreement with the low-coverage (

Formaldehyde Adsorption on the Anatase TiO2(101) Surface

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