Oxygen Vacancy-Induced Novel Low-Temperature Water Splitting

Article pubs.acs.org/JPCC

Oxygen Vacancy-Induced Novel Low-Temperature Water Splitting Reactions on FeO(111) Monolayer-Thick Film Lingshun Xu,†,‡,§ Zongfang Wu,†,‡,§ Wenhua Zhang,‡ Yuekang Jin,†,‡,§ Qing Yuan,†,‡,§ Yunsheng Ma,§ and Weixin Huang*,†,‡,§ †

Hefei National Laboratory for Physical Sciences at the Microscale, ‡CAS Key Laboratory of Materials for Energy Conversion, and Department of Chemical Physics, University of Science and Technology of China, Jinzhai Road 96, Hefei 230026, China

§

S Supporting Information *

ABSTRACT: We have used XPS, UPS, and TDS to comparatively study water chemisorption and reaction on stoichiometric FeO(111) monolayer-thick film on Pt(111), stoichiometric FeO(111) monolayer-thick islands on Pt(111), and FeO(111) monolayer-thick films with oxygen vacancies on Pt(111) at 110 K. On stoichiometric FeO(111) monolayer-thick film, water undergoes reversible molecular adsorption. On stoichiometric FeO(111) monolayer-thick islands on Pt(111), water dissociates at coordination-unsaturated Fe(II) sites of the FeO(111)−Pt(111) interface to form OH following H2O + FeCUS + FeO → FeCUS− OwH + FeOH in which Ow means O from H2O. Upon heating, H2 evolution occurs above 500 K. On FeO(111) monolayer-thick films with oxygen vacancies, water dissociates and molecularly chemisorbs to form a mixed adsorbate layer of H(a), OH, and H2O(a) following both H2O + Fe−Ovacancy + FeO → FeOwH + FeOH and H2O + 2 Fe−Ovacancy → FeOwH + H(a)−Fe−Ovacancy. Upon heating, besides the high-temperature H2 evolution, additional H2 desorption peaks appear simultaneously with the lowtemperature desorption features of adsorbed H2O(a), revealing novel low-temperature water splitting reactions. The formation of hydrated-proton surface species within a mixed adsorbate layer of H(a), OH, and H2O(a) on FeO(111) monolayer-thick films with oxygen vacancies is proposed to explain such novel low-temperature water splitting reactions. These results greatly enrich the surface chemistry of water on solid surfaces. film on Ru(0001),11 likely O2 evolution on Fe3O4(001) surface,12 and CeO2−x(111) thin film on Pt(111).13 Oxygen vacancies on the transition metal oxide surface have been recognized to strongly influence its reactivity. The Oterminated FeO(111) monolayer-thick film (denoted as FeO(111)/Pt(111)) and islands epitaxially grown on Pt(111)14−16 have recently received great attention as the model catalysts to understand the promotion effect of iron to Pt-based catalysts in several important catalytic reactions.17−24 FeO(111)/Pt(111) provides a very suitable model oxide surface to study the influence of oxygen vacancies on the reactivity of an oxide surface. Once formed, oxygen vacancies on FeO(111)/Pt(111) remain stable even at elevated temperatures because of the lack of the bulk-to-surface diffusion

1. INTRODUCTION Water adsorption on transition metal oxides has received considerable attention over the past few years due to its relevance to many important catalytic reactions including photocatalytic splitting of water, steam reforming reaction, and water gas shift reaction. Surface science study of water adsorption on well-defined transition metal oxide single crystals and thin films is the main approach for the fundamental understanding.1−6 Water chemisorption on transition metal oxide model surfaces can be classified into reversible molecular adsorption, reversible dissociation in which water is evolved on heating, and irreversible dissociation in which dissociation products (H2 and O2) are evolved on heating. Among a huge number of investigated systems,1−6 only several examples of irreversible water dissociation have been reported, including H2 evolution on O-terminated UO2(100)-c(2 × 2) surface,7 rutile TiO2(110) surface with surface oxygen vacancies,8 BaO monolayer films on W(110)9 or W(001),10 CeO2−x(111) thin © 2012 American Chemical Society

Received: July 18, 2012 Revised: September 2, 2012 Published: October 8, 2012 22921

dx.doi.org/10.1021/jp307104a | J. Phys. Chem. C 2012, 116, 22921−22929

The Journal of Physical Chemistry C

Article

Figure 1. Fe 2p and O 1s XPS spectra, and He II UPS spectra of Pt(111), FeO(111)/Pt(111), 0.68 ML FeO(111)/Pt(111), FeO0.88(111)/Pt(111), FeO0.76(111)/Pt(111), and FeO0.67(111)/Pt(111).

process of lattice oxygen that usually occurs in thick oxide films. FeO(111)/Pt(111) is rather inert under ultrahigh vacuum (UHV) conditions,16 but we found that employing atomic hydrogen could reproducibly create oxygen vacancies in FeO(111)/Pt(111) and further observed the oxygen vacancycontrolled reactivity of surface hydroxyls.20−22,25 Water reversibly and molecularly adsorbs on FeO(111)/ Pt(111).26−29 Knudsen et al. recently reported that water chemisorption on Fe3O2(111) film prepared by the reduction of FeO(111)/Pt(111) with atomic hydrogen shows a fully reversible dissociation nature at room temperature and reoxidizes the surface partly above 350 K.30 In this Article, we report a comparative study of water chemisorption and reaction on stoichiometric FeO(111) monolayer-thick film on Pt(111), stoichiometric FeO(111) monolayer-thick islands on Pt(111), and FeO(111) monolayerthick films with oxygen vacancies on Pt(111). Interestingly, we observed oxygen vacancy-induced novel surface chemistry of water on FeO(111) monolayer-thick film at low temperatures. The dissociation and molecular adsorption of water on FeOx(111)/Pt(111) surfaces (x < 1) at 110 K form a mixed layer of H(a), OH, and H2O(a). Upon heating, we observed the simultaneous desorption traces of H2 and H2O from the surface below 200 K. These results point to a novel mechanism for low-temperature irreversible dissociation of H2O on oxide thin film surfaces.

Pt(111) interfaces were prepared on Pt(111) by the evaporation of controlled amounts of high-purity iron (99.995%, Alfa Aesar China Co., Ltd.) onto Pt(111) at room temperature followed by the oxidation in 1 × 10−6 mbar O2 at 1000 K for 30 min.20−22,25 FeOx(111)/Pt(111) surfaces with oxygen vacancies were reproducibly prepared by repeated cycles of an exposure of stoichiometric FeO(111)/Pt(111) to 50 L atomic hydrogen at 110 K followed by flashing to 650 K.21 After each measurement, the FeOx(111)/Pt(111) model surfaces were reoxidized in O2 (PO2 = 5 × 10−8 mbar) at 1000 K for 10 min to restore the stoichiometric FeO(111)/ Pt(111) surface, which was then subjected to the same procedure for the reproducible preparation of model surfaces with oxygen vacancies. D2O (D > 99.9%, Sigma-Aldrich) and H2O (ultrapure, >18 MΩ) were purified by repeated freeze− pump−thaw cycles. Other high-purity gases were purchased from Nanjing ShangYuan Industry Factory and used as received. The purity of all reactants was checked by QMS prior to experiments. All exposures were reported in Langmuir (1 L = 1.0 × 10−6 Torr s) without corrections for the gauge sensitivity. During the TDS experiments, the sample was positioned ∼1 mm away from a collecting tube of the differentially pumped QMS, and the heating rate was 3 K/s. XPS spectra were recorded using Mg Kα radiation (hν = 1253.6 eV) with a pass energy of 20 eV, and UPS spectra were recorded using He II radiation (hν = 40.8 eV) with a pass energy of 5 eV.

2. EXPERIMENTAL SECTION All experiments were performed in a Leybold stainless-steel UHV chamber with a base pressure of 1.2 × 10−10 mbar.31 The UHV chamber was equipped with facilities for XPS, UPS, LEED, and differentially pumped TDS. The UHV chamber was also equipped with a QUAD-EV-S mini e-beam evaporator and a MGC75 thermal gas cracker. The Pt(111) single crystal purchased from MaTeck was mounted on the sample holder by two Ta wires spot-welded to the back side of the sample. The sample temperature could be controlled between 100 and 1473 K and was measured by a chromel-alumel thermocouple spotwelded to the backside of the sample. Prior to the experiments, the Pt(111) sample was cleaned by repeated cycles of Ar+ sputtering and annealing until LEED gave a sharp (1 × 1) diffraction pattern and no contaminants could be detected by XPS. Five model surfaces were prepared employing clean Pt(111) substrate. The stoichiometric FeO(111) monolayer film and FeO(111) monolayer-thick islands containing FeO(111)−

3. RESULTS AND DISCUSSION FeOx(111)/Pt(111) surfaces with various concentrations of oxygen vacancies were reproducibly prepared by repeated cycles of an exposure of stoichiometric FeO(111) monolayer film on Pt(111) to 50 L atomic hydrogen at 110 K followed by flashing to 650 K.21 The surface compositions of various FeOx(111)/Pt(111) surfaces were evaluated from their Fe 2p and O 1s XPS spectra (Figure 1). The O 1s binding energy of O 1s XPS spectra does not shift after the stoichiometric FeO(111)/Pt(111) surface experienced repeated cycles of treatment, but their peak intensities keep decreasing with the increasing number of repeated cycles, being 88%, 76%, and 67% of that of the stoichiometric FeO(111)/Pt(111) surface after one cycle, two cycles, and three cycles of treatment, respectively. The corresponding Fe 2p XPS spectra do not change much. These results demonstrate that FeOx(111)/ Pt(111) surfaces with 0.12, 0.24, and 0.33 ML oxygen vacancies, respectively, denoted as FeO0.88(111)/Pt(111), 22922



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Pt(111) is 0.68 ML, and the prepared FeO(111)/Pt(111) inverse model surface is denoted as 0.68 ML FeO(111)/ Pt(111). Figure 3 shows the TDS spectra of D2O and D2 following the exposure of 0.1 L D2O on various model surfaces at 110 K. Agreeing with previous results,1,26−28 only the D2O reversible molecular desorption peak was observed from Pt(111) and FeO(111)/Pt(111). On the 0.68 ML FeO(111)/Pt(111) surface, besides the D2O molecular desorption peak from both Pt and FeO(111) surfaces, a D2 desorption peak at 510 K was observed and could be assigned to the surface reaction of OD on FeOx(111) (x ≤ 1) surfaces.21,22,25 Thus, coordinationunsaturated Fe atoms (FeCUS) exposed at the Pt−FeO interface23 are capable of dissociating D2O to form OD at 110 K and eventually result in the irreversible water decomposition at high temperatures. On FeOx(111)/Pt(111) surfaces (x < 1), the D2O reversible molecular desorption peak appears at higher temperatures than that on the FeO(111)/ Pt(111) surface, and it shifts to higher temperatures with the increase of oxygen vacancy concentration, indicating the oxygen vacancy-enhanced interaction of D2O(a) with FeO(111) surfaces. We are now performing DFT calculations of water chemisorption and reaction on FeOx(111)/Pt(111) surfaces. The preliminary results show that the adsorption energy of H2O(a) on the oxygen vacancy site of FeOx(111)/Pt(111) surfaces is much larger than on the stoichiometric FeO(111)/ Pt(111) surface, supporting our experimental observations. Moreover, two D2 desorption peaks were also observed: one below 200 K (denoted as α peak) and the other above 450 K (denoted as γ peak). Yet no O2 desorption peak was observed. Thus, oxygen vacancies on the FeO(111)/Pt(111) surface can induce the irreversible water dissociation, producing H2 and reoxidizing the surface. With the increase of oxygen vacancy concentration, the γ-D2 peak formed by the surface reaction of OD on FeOx(111)/Pt(111)21,22 shifts to lower temperatures, while its peak intensity slightly decreases, implying that a high oxygen vacancy concentration facilitates this surface reaction. This agrees with our previous observation that the reactivity of hydroxyls on FeOx(111) monolayer films on Pt(111) is controlled by the concentration of oxygen vacancies.21 Interestingly, the α-D2 peak was found to appear simultaneously with the D2O reversible molecular desorption peak. This demonstrates that both desorption products result from the same surface process. To our knowledge, these results report for the first time the low-temperature irreversible water

FeO0.76(111)/Pt(111), and FeO0.67(111)/Pt(111), were successfully prepared. LEED patterns of various FeOx(111)/ Pt(111) surfaces (Figure 2) indicate that FeOx(111) (x < 1)

Figure 2. LEED patterns of Pt(111), FeO(111)/Pt(111), 0.68 ML FeO(111)/Pt(111), FeO0.88(111)/Pt(111), FeO0.76(111)/Pt(111), and FeO0.67(111)/Pt(111).

monolayer-thick films are geometrically similar to stoichiometric FeO(111) monolayer-thick film. The FeO0.67(111)/ Pt(111) surface with a p(2 × 2) LEED pattern is resistant to further reduction by atomic hydrogen at room temperature.21,25,30 A combined XPS, STM, and DFT calculation study shows that the reduction process of stoichiometric FeO(111)/Pt(111) surface by atomic hydrogen can be described as the gradual transformation of 3-fold O-coordinated Fe atoms into 2-fold O-coordinated Fe atoms.30,32 We have also measured the UPS spectra of FeO(111)/Pt(111) and FeO0.67(111)/Pt(111) (Figure 1). The results indicate that FeO(111)/Pt(111) and FeOx(111)/Pt(111) surfaces have similar electronic structures. The peak intensity of the O 1s XPS spectrum of FeO(111) islands on Pt(111) amounts to 68% of that of stoichiometric FeO(111) monolayer film on Pt(111) (Figure 1). The corresponding Fe 2p XPS spectrum of FeO(111) islands on Pt(111) is much weaker than those of FeOx(111) and FeO(111) monolayer films on Pt(111). We have also used CO and H2 TDS experiments to titrate the coverage of bare Pt(111) surface on this FeO(111)/Pt(111) inverse model surface (results not shown) to be ∼32% of clean Pt(111) surface. Thus, the coverage of FeO(111) islands on

Figure 3. (A) D2O and (B) D2 TDS spectra following an exposure of 0.1 L D2O on (a) Pt(111), (b) 0.68 ML FeO(111)/Pt(111), (c) FeO(111)/ Pt(111), (d) FeO0.88(111)/Pt(111), (e) FeO0.76(111)/Pt(111), and (f) FeO0.67(111)/Pt(111) at 110 K. 22923



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the saturated D(a) coverage on Pt(111) of 1 ML, we estimated the coverages of total surface species yielding the α+β and whole D2 desorption peaks (Table 1), and the maximum yielding the whole D2 desorption peaks following a 0.3 L D2O exposure only amounts to ∼0.029 ML. Thus, only a very small amount of D2O undergoes the irreversible dissociation under our investigated conditions. We observed the H−D exchange process during the D2O adsorption that is representatively demonstrated by the case of 0.1 L D2O exposure (Figure S1E). D2O, HDO, and H2O desorb simultaneously, and HD desorbs with D2, but few H2 desorbs. These observations also indicate the dissociative chemisorption of D2O at 110 K. Figure 5 shows the O 1s XPS spectra following various exposures of D2O on FeO0.67(111)/Pt(111) at 110 K and the

dissociation accompanied by the reversible water desorption process following water adsorption on an oxide surface. Evidently, such a low-temperature water splitting reaction on FeO(111) monolayer-thick surfaces on Pt(111) is induced by oxygen vacancies and has nothing to do with the Pt−FeO interface. The adsorption and reaction of water on FeO0.67(111)/ Pt(111) at 110 K were thus studied in detail. Figure 4 shows

Figure 4. (A) D2O and (B) D2 TDS spectra following exposures of indicated exposures of D2O on FeO0.67(111)/Pt(111) at 110 K.

the TDS spectra of D2O and D2 following various exposures of D2O on FeO0.67(111)/Pt(111) at 110 K. The corresponding TDS spectra plotted individually are presented in Figure S1. At the lowest exposure (0.002 L D2O), the γ-D2 desorption peak is clear, and a broad and weak desorption feature between 200 and 400 K could also be identified for both D2O and D2. With the D2O exposure increasing, the α-D2O desorption peak appears and grows, accompanied by the α-D2 desorption peak. The α-D2O and α-D2 desorption peaks saturate when the D2O exposure reaches 0.2 L. A new β-D2O desorption peak at 165 K evolves at D2O exposures larger than 0.2 L and does not saturate, corresponding to the desorption of multilayer D2O(a) from the surface. Interestingly, similar to the α-D2O desorption peak, the β-D2O desorption peak is also accompanied by a β-D2 desorption peak. Except the α- and β-D2O and D2 desorption features, other D2O and D2 desorption traces were previously observed in the TDS spectra following exposures of atomic D on various FeOx(111)/Pt(111) surfaces and resulted from surface reactions of OD and D(a).21 A noteworthy issue is the amount of desorbed D2. As compared to the D2 TDS spectrum from saturating D(a)/Pt(111) (Figure S1H)33 and assuming

Figure 5. (A) O 1s XPS spectra and (B) corresponding coverages of various oxygen species following exposures of indicated amounts of D2O on FeO0.67(111)/Pt(111) at 110 K.

variation of coverage of different oxygen species as a function of D2O exposure acquired by the peak-fitting analysis. The individual O 1s XPS spectra that clearly demonstrate the fitting results at low exposures are presented in Figure S2, and the coverages of various surface species are summarized in Table 1. Following the lowest exposure of 0.002 L D2O (Figure S2A), two new O 1s XPS peaks appear at 531 and 533.2 eV that could be assigned to OD and α-D2O(a), respectively; meanwhile, the FeO XPS component weakens. The formation of OD clearly demonstrates the dissociative chemisorption of D2O on the oxygen vacancy sites of the surface. We found that the coverage of formed OD is nearly twice the decreased coverage of FeO (Table 1), implying that the dissociative

Table 1. Coverages (ML) of Various Surface Species Following Various Exposures of D2O on FeO0.67(111)/Pt(111) at 110 K Estimated from XPSa coverages estimated from XPS exposure (L) 0 0.002 0.005 0.02 0.05 0.1 0.2 0.3

coverages estimated from TDS

FeO

ΔFeO

OD

α-D2O(a)

β-D2O(a)

D(a)

OD/ΔFeO

surface speciesb

surface speciesc

0.67 0.657 0.641 0.64 0.639 0.64 0.641 0.64

0 0.013 0.029 0.03 0.031 0.03 0.029 0.03

0 0.026 0.056 0.128 0.196 0.23 0.29 0.296

0 0.017 0.037 0.093 0.216 0.359 0.608 0.616

0 0 0 0 0 0.093 0.154 0.562

0 0 0 0.068 0.134 0.17 0.232 0.236

2 1.93 4.27 6.32 7.67 10 9.87

0 0.001 0.001 0.004 0.006 0.014 0.018 0.029

0 0 0 0.002 0.004 0.012 0.015 0.025

The coverage of D(a) was estimated by (OD − 2ΔFeO). The coverages of surface species yielding the α+β and whole D2 desorption peaks in the TDS spectra are also included. These values were estimated by comparing to the D2 TDS spectrum from saturating D(a)/Pt(111) and assuming the saturated D(a) coverage on Pt(111) of 1 ML. bSurface species yielding whole D2 desorption peaks. cSurface species yielding α+β D2 desorption peaks. a

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Figure 6. O 1s XPS spectra after annealing FeO0.67(111)/Pt(111) exposed to (A) 0.002 L, (B) 0.05 L, and (C) 0.3 L D2O at 110 K to indicated temperatures.

Table 2. Coverages (ML) of Various Surface Species during the Annealing Process of FeO0.67(111)/Pt(111) Exposed to Various Exposures of D2O at 110 Ka exposure (L)

temp (K)

FeO

OD

α-D2O(a)

β-D2O(a)

D(a)

ΔFeO

OD/ΔFeO (ML)

0.002

110 175 210 300 600 110 175 210 300 600 110 175 210 300 600

0.657 0.642 0.64 0.641 0.668 0.640 0.63 0.634 0.64 0.67 0.64 0.63 0.632 0.634 0.067

0.026 0.057 0.061 0.06 0 0.146 0.22 0.129 0.072 0 0.296 0.296 0.275 0.175 0

0.017 0 0 0 0 0.226 0.17 0 0 0 0.616 0.445 0 0 0

0 0 0 0 0 0 0 0 0 0 0.562 0.096 0 0 0

0 0 0 0 0 0.086 0.14 0.057 0.012 0 0.236 0.216 0.199 0.103 0

0.013 0.028 0.03 0.029 0.002 0.03 0.04 0.036 0.03 0 0.03 0.04 0.038 0.036 0

2.04 2.05 2.03 2.07

0.05

0.3

a

4.87 5.5 3.58 2.4 9.87 7.4 7.24 4.89

The coverage of D(a) was estimated by (OD − 2ΔFeO).

could be estimated by the OD coverage subtracted by twice the decreased FeO coverage (OD − 2ΔFeO) and is also summarized in Table 1. When the D2O exposure exceeds 0.05 L, the O 1s binding energy of α-D2O(a) gradually shifts downward and reaches 532.5 eV after the exposure of 0.3 L D2O. Thus, more and more D2O molecules chemisorb on the stoichiometric FeO(111) surface sites of the surface. A new O 1s component appears at 533.8 eV after an exposure of 0.1 L D2O that can be assigned to multilayer D2O(a) (β-D2O(a)). It is noteworthy that the desorption peak of β-D2O(a) was observed in the TDS spectra after an exposure of 0.2 L D2O. This implies that the continuous adsorption of D2O in the residual gas of the UHV chamber should occur during the XPS measurement after the D2O exposure. With the increase of the D2O exposure, the β-D2O (a) component grows, and its binding energy shifts downward and reaches 533.3 eV after the exposure of 0.3 L D2O. The OD and α-D2O(a) reach the saturation with the saturating coverage, respectively, of 0.3 and 0.61 ML after the exposure of 0.2 L D2O. The OD saturating coverage approaches the oxygen vacancy coverage (0.33 ML) on the FeO0.67(111)/Pt(111) surface; therefore, almost all αD2O(a) chemisorbs on the stoichiometric FeO(111) surface sites at the saturating coverage, although α-D2O(a) initially chemisorbs on the oxygen vacancy sites. This implies that αD2O(a) initially chemisorbed on the oxygen vacancy sites

chemisorption of D2O follows the equation of D2O + Fe− Ovacancy + FeO → FeOwD + FeOD, in which Ow means O from D2O. The O 1s binding energy of α-D2O(a) is higher than that (532.5 eV) of D2O(a) on stoichiometric FeO(111)/Pt(111) (Figure S3). This indicates that α-D2O(a) at this coverage chemisorbs on the oxygen vacancy sites. When the D2O coverage increases to 0.005 L, the OD and α-D2O(a) components grow at the expense of the FeO component. The O 1s binding energy of OD shifts upward to 531.2 eV, while that of α-D2O(a) shifts downward to 533 eV (Figure S2B). This might be taken as an indication for the formation of a hydrogen bond within the mixed OD and α-D2O(a) layer on the surface.34,35 The coverage of formed OD was still found to be nearly twice the decreased coverage of FeO (Table 1), implying that the dissociative chemisorption of D2O still follows the equation of D2O + Fe−Ovacancy + FeO → FeOwD + FeOD. The OD and α-D2O(a) components keep growing with the further increase of the D2O exposure, but the FeO component does not weaken anymore (Figure 5B and Table 1). This demonstrates that FeO is no longer involved in the formation of OD. One likely surface reaction is D2O + 2Fe− Ovacancy → FeOwD + D(a)−Fe−Ovacancy in which D(a) chemisorbs on the coordinated-unsaturated Fe(II) sites exposed on the surface.21,30 Parkinsin et al.36 reported the formation of HFeOH species upon reaction of Fe atoms with condensed water. The coverage of formed D(a) on the surface 22925



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Figure 7. Schematic illustration for the adsorption and surface reactions of water on FeOx(111)/Pt(111) (x < 1) surfaces at 110 K. Green, purple, red, blue, and white circles represent Pt, Fe, O in FeO, O in H2O, and H atoms, respectively.

elevated temperatures are similar to those in the case of FeO0.67(111)/Pt(111) exposed to 0.05 L D2O. Thus, on the basis of above TDS and XPS results, we could summarize the adsorption and surface reactions of D2O on FeOx(111)/Pt(111) (x < 1) surfaces at 110 K (Figure 7). D2O majorly undergoes the reversible dissociation that follows D2O + Fe−Ovacancy + FeO ⇆ FeOwD + FeOD at very low coverages and D2O + 2Fe−Ovacancy ⇆ FeOwD + D(a)−Fe−Ovacancy at high coverages. Meanwhile, a very small amount of water undergoes the irreversible dissociation producing D2 and reoxidizing the surface. However, the coverage increase of FeO induced by the reoxidation was not observed by XPS, which might be due to the very low values (∼0.0145 ML after the exposure of 0.3 L D2O). The D2 desorption peaks above 200 K arise from reactions of OD and D(a) on the surface.21 Particularly of interest are the α- and β-D2 desorption peaks desorbed simultaneously with water when D2O(a), OD, and D(a) coexist on the surface. Such experimental results have never been previously observed on oxide and oxide thin film surfaces and point to oxygen vacancy-induced novel surface reactions resulting in low-temperature water splitting. H(a) and H2O(a) coadsorbed on Pt surfaces were previously observed to give the simultaneous H2 and H2O desorption peaks that were attributed to a mixture of hydrated-proton surface species (H+−(H2O(a)n)) formed by the incorporation of H(a) in H2O(a) layers.37−39 D2O dissociatively and molecularly adsorbs on FeOx(111)/Pt(111) surfaces (x < 1) at 110 K, forming a mixed layer of D(a), OD, and D2O(a). In addition to XPS results, UPS results (Figure 8) also indicate the formation of a hydrogen-bonding network within OD and D2O(a) on the surface. After an exposure of 0.05 L D2O on FeO0.67(111)/Pt(111) at 110 K, valence-band features could be clearly observed for D2O(a) but not for OD, although XPS detected OD and D2O(a) signals. The OD valence-band features appear in the UPS spectrum only after annealing at 210 K when no D2O(a) exists on the surface. Thus, OD in the OD + D2O(a) layer exhibits D2O(a)-like valence-band features instead of its own characteristic ones, strongly indicating the formation of the hydrogen-bonding network. Within such a hydrogen-bonding network containing D(a), OD, and D2O(a), weakly chemisorbed D(a) likely interacts with D2O(a) via hydrogen bonding to form D+−D2O(a)n surface species (Figure 7). Protons have been demonstrated to be mobile within the hydrogen-bonding network;40−42 thus D+ in D+−

should further undergo the dissociation reaction with the increase of the total surface coverage. Figure 6 shows the O 1s XPS spectra during the annealing process of FeO0.67(111)/Pt(111) exposed to different amounts of D2O at 110 K. The coverages of various surface species during the annealing process acquired by the peak-fitting analysis are summarized in Table 2. Annealing FeO0.67(111)/ Pt(111) exposed to 0.002 L D2O to 175 K leads to the disappearance of the α-D2O(a) component and the growth of the OD component. The increase of the OD coverage is nearly twice the decreased coverage of α-D2O(a) and FeO (Table 2), indicating that α-D2O(a) undergoes the dissociation reaction following the equation of α-D2O(a) + FeO → FeOwD + FeOD. This also supports our argument that α-D2O(a) initially chemisorbs on the oxygen vacancy sites of the surface because water chemisorbed on the stoichiometric FeO(111) surface does not decompose upon heating. The OD feature does not disappear until after the annealing at 600 K. The surface reactions of OD reasonably give rise to the D2O and D2 desorption traces observed in the corresponding TDS spectra. Annealing FeO0.67(111)/Pt(111) exposed to 0.05 L D2O to 175 K also leads to the attenuation of the α-D2O(a) component and the growth of the OD component, demonstrating the dissociation of some α-D2O(a) molecules. Inferred from the relative changes in the coverages of OD, αD2O(a), and FeO (Table 2), we found that the dissociation of α-D2O(a) mostly follows the likely equation of α-D2O(a) → FeOwD + D(a). Upon further annealing to 210 K, the αD2O(a) component disappears, corresponding to the α-D2O(a) desorption peak; meanwhile, the coverages of OD and D(a) also decrease (Table 2). The OD feature disappears after the annealing at 600 K. As revealed by the TDS results, only a very small amount of OD and D(a) undergoes the surface reactions to produce D2; therefore, the major surface reactions are those producing water. Upon annealing FeO0.67(111)/Pt(111) exposed to 0.3 L D2O with saturating OD and α-D2O(a) to 175 K, the α- and βD2O(a) components greatly weaken, the D(a) coverage slightly decreases, but the OD coverage does not change (Table 2). These indicate that α- and β-D2O(a) undergo desorption instead of dissociation. The slight decrease in the D(a) coverage corresponds to the α- and β-D2 desorption peaks. Surface reactions occurring upon annealing the surface at 22926



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reaction of D2O on 0.68 ML FeO(111)/Pt(111) surface with FeCUS at 110 K form the OD + D2O(a) mixed layer without D(a), and reasonably the low-temperature water splitting does not occur. This might also apply for the case of water adsorption on Fe3O4(111) surface exposing both Fe and O ions.16,43,44 Water both dissociatively and molecularly chemisorbs on Fe3O4(111) to form the OH+H2O layer,16,44 and DFT calculations suggest the formation of OH3+−OH within the adlayer,42 but only reversible water dissociation occurs.16,44 The different reactivities between FeCUS on the FeO(111)− Pt(111) interface of 0.68 ML FeO(111)/Pt(111) surface and FeCUS on FeOx(111)/Pt(111) surfaces toward water can be reasonably attributed to their different local chemical environments. The underlying mechanisms need further investigations. A noteworthy issue is that some processes occurring during water adsorption and reaction on FeOx(111)/Pt(111) surfaces, for examples, the formation of H+−(H2O(a))n from H(a) and H2O(a) and the decomposition of H+−(H2O(a))n into H2O and H2, are accompanied by the creation of extra charges. We propose that these extra charges might be balanced via the Pt(111) substrate underneath the FeOx(111) monolayer-thick film. Meanwhile, the charge distributions at an ultrathin oxide film supported by a metal substrate that are closely related to its chemical reactivity have been demonstrated to be different from those at bulk oxides and influenced by the presence of defects.18,45−48 The most significant finding of our results is the formation of hydrated-proton surface species after the adsorption of H2O on FeOx(111)/Pt(111) surfaces that decompose to produce H2 at low temperatures. This reveals a novel low-barrier reaction pathway for the catalyzed H2 production from water catalyzed by solid surfaces. Combining previous results on Pt(111)37−39 and the present results, the formation of hydrated-proton surface species on solid catalyst surfaces only requires the formation of a hydrogen-bonding network consisting of H(a) or H+, OH, and H2O(a). These requirements can be quite facilely satisfied for catalytic reactions in the aqueous solution; therefore, hydrated-proton surface species might be generally present on the surface of catalysts used for these catalytic reactions including photosplitting of water and involved in these catalytic reactions.

Figure 8. He II UPS spectra of clean FeO0.67(111)/Pt(111) surface (a), FeO0.67(111)/Pt(111) surface exposed to 0.05 L D2O at 110 K (b), followed by annealing at 175 K (c), 210 K (d), 300 K (e), and 600 K (f).

(D2O(a)n) surface species formed within the chemisorbed layer could also migrate into the physisorbed D2O(a) multilayer to form D+−(D2O(a)n) surface species. Upon heating, D+− (D2O(a)n) surface species decompose together with the desorption of D2O(a) from the surface, and the incorporated protons desorbs simultaneously as D2. The formation of the hydrogen-bonding network might also be the driving force for the dissociation of α-D2O(a) initially chemisorbed on the oxygen vacancy sites as the total surface coverage increases that was observed by XPS. This surface process could serve as another likely pathway to form the hydrated surface species: α‐D2 O(a) + n D2 O → Ow D + D+ −( D2 O(a))n

For comparison, we also studied the adsorption and surface reactions of D2O on 0.68 ML FeO(111)/Pt(111) surface with FeCUS at 110 K by XPS (Figure 9). After an exposure of 0.05 L D2O, both OD and D2O(a) form on the surface. The coverage of OD is nearly twice the decreased coverage of FeO, implying that the dissociative chemisorption of D2O on FeCUS exposed at the Pt−FeO interface follows the equation of D2O + FeCUS + FeO → FeCUS−OwD + FeOD. Annealing the surface to 175, 210, and 600 K results in the desorption of D2O(a), the surface reaction of FeCUS−OwD + FeOD → D2O + FeCUS + FeO, and the surface reaction of FeCUS−OwD + FeOD → D2 + FeCUS−O + FeO, respectively. These results agree well with previous TDS results (Figure 3). Thus, the adsorption and surface

4. CONCLUSIONS By a comparative study of water chemisorption and reaction on stoichiometric FeO(111) monolayer-thick film on Pt(111), stoichiometric FeO(111) monolayer-thick islands on Pt(111), and FeO(111) monolayer-thick films with oxygen vacancies on

Figure 9. O 1s XPS spectra after 0.05 L D2O exposure on 0.68 ML FeO(111)/Pt(111) at 110 K followed by annealing at indicated temperatures and the corresponding coverages of various surface species. 22927



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Pt(111) at 110 K, we have successfully revealed a novel oxygen vacancy-induced mechanism for low-temperature water splitting on FeOx(111) monolayer-thick film (x < 1) via the hydrated-proton surface species. Water dissociatively and molecularly adsorbs on FeOx(111)/Pt(111) surfaces (x < 1) at 110 K, forming a mixed layer of H(a), OH, and H2O(a) within which the hydrated-proton surface species form. Upon heating, the hydrated-proton surface species decompose together with the desorption of H2O(a) from the surface, and the incorporated protons desorb simultaneously as H2. These findings broaden the surface chemistry of water on solid surfaces and highlight the role of oxygen vacancy in controlling the reactivity of oxide surfaces.

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ASSOCIATED CONTENT

S Supporting Information *

TDS spectra plotted individually following various exposures of D2O on FeO0.67(111)/Pt(111) at 110 K shown in Figure S1, O 1s XPS spectra plotted individually following various exposures of D2O on FeO0.67(111)/Pt(111) at 110 K shown in Figure S2, D2O TDS spectra, O 1s XPS spectra, and corresponding coverages of various oxygen species following indicated exposures of D2O on stoichiometric FeO(111)/Pt(111) at 110 K shown in Figure S3. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (21173204), the National Basic Research Program of China (2013CB933104, 2010CB923301), the Chinese Academy of Sciences, MOE Fundamental Research Funds for the Central Universities, and the MPG-CAS partner group program.

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REFERENCES

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Oxygen Vacancy-Induced Novel Low-Temperature Water Splitting

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