Probing equilibrium of molecular and deprotonated water on TiO2(110) Zhi-Tao Wanga,b,1, Yang-Gang Wangb,c,1, Rentao Mub,c,1, Yeohoon Yoonb,c, Arjun Dahala,b, Gregory K. Schenterb,d, Vassiliki-Alexandra Glezakoub,c, Roger Rousseaub,c,2, Igor Lyubinetskyb,c,2,3, and Zdenek Dohnálekb,c,2 aMicroscopy Group, Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, WA 99352; bInstitute for Integrated Catalysis, Pacific Northwest National Laboratory, Richland, WA 99352; cCatalysis Group, Physical Sciences Division, Physical and Computational Sciences Directorate, Pacific Northwest National Laboratory, Richland, WA 99352; and dChemical Physics & Analysis Group, Physical Sciences Division, Physical & Computational Sciences Directorate, Pacific Northwest National Laboratory, Richland, WA 99352 Edited by Martin Gruebele, University of Illinois at Urbana–Champaign, Urbana, IL, and approved January 10, 2017 (received for review August 17, 2016) Understanding adsorbed water and its dissociation to surface To address the adsorption configuration of isolated water hydroxyls on oxide surfaces is key to unraveling many physical molecules, extremely low coverages are required. As such, the and chemical processes, yet the barrier for its deprotonation has ensemble-averaged methods generally do not possess sufficient never been measured. In this study, we present direct evidence for sensitivity and STM is the method of choice. Although the ap- water dissociation equilibrium on rutile-TiO2(110) by combining su- pearance of water-related features is suggestive of molecular personic molecular beam, scanning tunneling microscopy (STM), binding, indirect evidence strongly indicates that water monomers and ab initio molecular dynamics. We measure the deprotonation/ can easily access the dissociated configuration (18, 19). For ex- protonation barriers of 0.36 eV and find that molecularly bound ample, in the water-assisted diffusion of bridging hydroxyl hy- water is preferred over the surface-bound hydroxyls by only drogen (18) and Ti-bound oxygen adatoms (19), water monomer 0.035 eV. We demonstrate that long-range electrostatic fields ema- dissociation represents a key step in the proposed mechanism. nating from the oxide lead to steering and reorientation of the This is further supported by the wealth of theoretical studies that molecules approaching the surface, activating the O–H bonds and yield very close adsorption energies (within ∼0.1 eV) for molec- inducing deprotonation. The developed methodology for studying ularly and dissociatively bound water monomers (14–18). metastable reaction intermediates prepared with a high-energy To address this topic, we have constructed a unique in- molecular beam in the STM can be readily extended to other sys- strument that allows for in situ molecular beam scattering studies tems to clarify a wide range of important bond activation processes. directly under the STM tip, an instrument combination that has hitherto proven difficult to achieve (26–28). In this study, we adsorbate dynamics | water | dissociative adsorption | titanium dioxide | used hyperthermal water beams with variable energy and probed kinetic barriers the probability of the formation of metastable binding configu- rations on TiO2(110) at low substrate temperatures (∼80 K). On ater is ubiquitous in the environment and, as such, the metals, such molecular beam studies have proven indispensable CHEMISTRY Wnature of its interactions with interfaces can determine the for the understanding of dynamic factors controlling dissociation outcome of a broad range of processes that include wetting, processes such as the excitation of vibrational modes, energy dissolution, precipitation, phase transformation, corrosion, and flow, and steering (29–31). We demonstrate that the observed catalytic and environmental reactions (1–7). In this regard, the metastable configurations are composed of terminal and bridging relative stability of molecularly and dissociatively bound species can be of critical importance with the preferred configuration Significance being controlled by many factors including surface structure, acid/base properties, defects, impurities, water coverage, and Understanding how water binds and dissociates on surfaces temperature (8–13). For oxides in particular, the relative stability has broad implications in a vast range of physical and chemical of molecularly and dissociatively bound water has been widely processes. The relative stability of molecularly and dissociatively debated even on the simplest, low-index surfaces. bound water has been debated for decades on many oxide Here we focus on resolving the fundamental question of water surfaces, but it has never been successfully measured. Our study describes unique instrumentation, direct measurements, and a binding on rutile TiO2(110), one of the most studied oxide sur- faces, which is often used as a prototype for reducible oxide state-of-the-art computation and theory approach that yield a detailed kinetic and dynamic description of water deprotona- surfaces and a model for understanding photocatalytic water tion equilibrium on TiO (110), a prototypical surface commonly splitting (3, 14–17). Interestingly, despite the overwhelming 2 used in mechanistic studies of photocatalytic water splitting. wealth of literature, the nature of water adsorption and dissoci- This unique study demonstrates that the molecularly bound ation on nondefect titanium sites has been disputed for decades water on TiO (110) is preferred over the surface-bound hy- – 2 and to date remains unsettled (3, 14 17). The underpinning dif- droxyls by only 0.035 eV. ficulty in resolving this debate is that it is practically impossible to prepare stoichiometric TiO2(110) surfaces. As such, bridging Author contributions: G.K.S., R.R., I.L., and Z.D. designed research; Z.-T.W., Y.-G.W., R.M., hydroxyl groups formed by water dissociation in oxygen vacancy Y.Y., A.D., and R.R. performed research; Y.-G.W., R.M., G.K.S., V.-A.G., R.R., I.L., and Z.D. defects interfere with determining the extent of dissociation on analyzed data; and V.-A.G., R.R., I.L., and Z.D. wrote the paper. regular Ti sites (3, 14–19). A number of recent studies by a variety The authors declare no conflict of interest. of techniques including X-ray photoelectron spectroscopy (XPS) This article is a PNAS Direct Submission. 1 (20), infrared reflection absorption (21), photoelectron diffrac- Z.-T.W., Y.-G.W., and R.M. contributed equally to this work. 2 tion (PhD) (22), and scanning tunneling microscopy (STM) (23– To whom correspondence may be addressed. Email: [email protected], igor. [email protected], or [email protected]. 25) arrived at conflicting conclusions. Whereas the XPS and PhD 3Present address: School of Chemical, Biological and Environmental Engineering, Oregon studies concluded partial dissociation of water in the hydrogen- State University, Corvallis, OR 97331. bonded chains on Ti sites at higher coverages (20, 22), others are This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. in favor of molecular bonding (21, 25). 1073/pnas.1613756114/-/DCSupplemental. www.pnas.org/cgi/doi/10.1073/pnas.1613756114 PNAS | February 21, 2017 | vol. 114 | no. 8 | 1801–1805 Downloaded by guest on October 1, 2021 hydroxyl pairs that can be converted back to molecularly bound We further probe the preparation of metastable species by waterathighertemperatures(>100 K). In parallel, theoretical colliding water molecules of high-incident energy (1.3 eV in Fig. protocols were developed to reveal mechanistic details, guide ex- 1D). In contrast to the low-incident energy experiment (Fig. 1B), periments, and provide data interpretation. We accurately de- two distinct types of features (65% H2O and 35% P) are observed. termined the equilibrium distribution and the interconversion The P features are round and also centered on Ti5c sites. High energy barriers. The analysis reveals that the dissociated water contrast is needed (Fig. 1D, Left) to clearly discern their differ- configuration is only 0.035 eV higher in energy. A unique finding ences from H2O. The line profiles in the inset further reveal that of this study is that long-range (5–10-Å) electrostatic coupling the P features are distinctly smaller and ∼30% less intense than between the incident molecules and the oxide surface results in a H2O. Whereas in the area shown in Fig. 1B the P features are not strong steering and reorientation before collision. present, they comprise ∼5% of the total population at low H2O Results and Discussion incident energy. The large-scale images illustrating the overall distribution of the species at both low- and high-incident energies The STM images of clean TiO2(110) composed of rows of to- are shown in SI Appendix, section S1 and Fig. S1. pographically low-lying fivefold-coordinated titanium ions (Ti5c, The appearance of the P features is surprising, as heuristically, imaged bright) and high-lying bridging oxygen ions (Ob, imaged pairs of two bright features (HO and HO ) due to water disso- A C t b dark) are shown in Fig. 1 and . A small fraction of Obsis ciation are expected. To further interrogate the chemical makeup missing, resulting in vacancies (V s) (15). O of the P species, we carry out temperature-dependent experiments Fig. 1B shows the result after dosing H O with a room-tem- 2 for water with incident energy of 1.3 eV (Fig. 2). These experi- perature effusive molecular beam (incident energy of 0.06 eV) ments allow us to follow the thermal equilibration of the HOt and on the TiO2(110) at 80 K. In all STM experiments, H2O coverage was kept very low (∼0.05 ML) to keep the observed molecules HOb species with H2O. In the first type of experiment, we image continuously the isolated. Only one type of feature, corresponding to water same area while the sample temperature is increased from 111 to monomers centered on the Ti5c rows (17, 18), is seen. Their uniformity is demonstrated in a high-contrast image (Fig. 1B, 114 K (Fig. 2A). The conversion of one P feature (yellow circle) Right) and by the reproducibility of the line profiles (Fig.