Nonthermal Water Splitting on Rutile TiO2 - ACS Publications

Feb 20, 2009 - The electron-stimulated reactions leading to H2 and O2 and the ... coverage of water on the Ti4+ sites (5.2 × 1014 molecules/cm2) is t...
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J. Phys. Chem. C 2009, 113, 4451–4460

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Nonthermal Water Splitting on Rutile TiO2: Electron-Stimulated Production of H2 and O2 in Amorphous Solid Water Films on TiO2(110) Nikolay G. Petrik and Greg A. Kimmel* Pacific Northwest National Laboratory, Mail Stop K8-88, P.O. Box 999, Richland, Washington 99352 ReceiVed: June 6, 2008; ReVised Manuscript ReceiVed: NoVember 12, 2008; In Final Form: January 22, 2009

The electron-stimulated reactions leading to H2 and O2 and the electron-stimulated desorption (ESD) of H2O from 0 - 60 ML films of amorphous solid water (ASW) adsorbed on TiO2(110) are investigated as a function of film thickness and isotopic composition at 100 K. For 100 eV incident electrons, both the H2 and O2 ESD yields have maxima when the ASW coverage is ∼20 monolayers (ML), while the H2O ESD yield increases monotonically with water coverage. All the products reach a coverage-independent yield above 40-50 ML. Experiments using layered films of H2O and D2O demonstrate that the molecular hydrogen is produced in reactions that occur preferentially at or near both the ASW/TiO2 interface and the ASW/vacuum interface. However, electronic excitations or ionic defects created within the interior of the ASW films by the energetic electrons can subsequently migrate to the interfaces where they initiate reactions. Electron irradiation of ASW films results in the formation of bridge-bonded hydroxyls on TiO2(110). These hydroxyls do not contribute to the H2 produced near the ASW/TiO2 interface. Instead, the results suggest that this H2 is produced from a stable precursor, trapped on or near the substrate. The proposed mechanism for the H2 production near the ASW/TiO2(110) interface is supported by a kinetic model that semiquantitatively reproduces the main features of the nonthermal reactions. I. Introduction TiO2 is an important catalyst for a variety of applications including the photocatalytic destruction of organic pollutants,1 as well as potentially for photocatalytic water splitting to produce hydrogen.2-4 The applications of TiO2 have motivated extensive research on the thermal and photon-stimulated reactions on single crystals, particles, and aqueous suspensions.1,5 Since many of the actual and potential uses of TiO2 for catalysis involve aqueous solutions, the thermal and nonthermal reactions of water with TiO2 are particularly important. These reactions have been investigated with a wide variety of techniques including many UHV-based, surface science techniques. However, despite intensive scrutiny, fundamental aspects of the reactions of water and other molecules on TiO2 surfaces remain to be addressed. The (110) surface is the most stable crystal face of rutile TiO2, and water adsorbs molecularly on a defect-free (110) surface.5,6 The surface is composed of alternating rows of 5-fold coordinated Ti4+ atoms and 2-fold coordinated bridge-bonded oxygen (BBO) atoms, along with fully coordinated oxygen atoms in the same plane as the Ti4+ atoms. Water dissociatively adsorbs in vacancies in the bridging oxygen rows, Ov, to form two bridging hydroxyl groups (OHb).5-11 By convention, a saturation coverage of water on the Ti4+ sites (5.2 × 1014 molecules/cm2) is typically defined as the “water monolayer” (MLTi) on TiO2(110). To facilitate comparison with previously reported results for electron-stimulated reactions in ASW on Pt(111), we will present water coverages, θ, in “ice monolayers” where 1 ML ) 1.1 × 1015 molecules/cm2 (corresponding to a “bilayer” in the (001) plane of hexagonal crystalline ice).12 Note that 2 MLTi ≈ 1 ML. * To whom correspondence [email protected].

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Titania is widely explored as a promising catalyst for photocatalytic splitting of water to produce hydrogen. However, water adsorbed on clean TiO2 surfaces cannot be efficiently split into H2 and O2 with light whose energy is above the band gap for TiO2 (Eg ≈ 3.2 eV for rutile) but below the first excitedstate in water (∼7.5 eV).1 As result, there is a lack of fundamental investigations of nonthermal dissociation of water on the surface of clean TiO2.5,6,9 On the other hand, energetic electrons (e.g., 100 eV) efficiently initiate reactions in thin water films (θ e 3 MLTi) adsorbed on TiO2(110) resulting in, for example, dissociation into H and OH.13 Recent studies of the electron-stimulated reactions in multilayer ASW films grown on Pt(111) show that for coverages up to ∼40-50 ML, reactions localized at or near the ASW/Pt interface play an important role.14-17 For example, experiments with layered D2O and H2O films showed that molecular hydrogen is produced preferentially at or near the ASW/vacuum and ASW/Pt interfaces.15,16 The results demonstrated that the H2 production is not due to diffusion of atomic hydrogen or hydrogen-containing molecules to the interfaces. Instead, the results showed that electronic excitations or ionic defects (such as H3O+ or OH-)12 produced by the incident electrons in the bulk subsequently diffused to either interface where they initiated reactions.16 Since the structure of ASW/metal oxide interfaces and the chemical activity of metal oxide surfaces are very different from metals, studies of the electron-stimulated reactions in ASW films on an oxide surface should help elucidate the role of the substrate in these nonthermal reactions. Here, we investigate the electron-stimulated reactions in amorphous solid water (ASW) films on TiO2(110) leading to the production of H2 and O2 and the desorption of H2O as function of water coverage, θ, and isotopic composition. For 100 eV incident electrons, H2 and O2 ESD yields have maxima at θ ≈ 20 ML, while the H2O ESD yield increases monotonically with water coverage. All the products reach a coverage-

10.1021/jp805013b CCC: $40.75  2009 American Chemical Society Published on Web 02/20/2009

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Petrik and Kimmel

independent yield for θ > 40-50 ML. Experiments using layered films of H2O and D2O demonstrate that molecular hydrogen is produced in reactions that occur preferentially at or near both the ASW/TiO2 interface and the ASW/vacuum interface, but not in the interior of the ASW films. The energy for the reactions, however, is absorbed from the incident electrons in the interior of the films creating electronic excitations and/or ionic defects which subsequently migrate to the interfaces where they drive the reactions. The TiO2(110) surface becomes hydrogenated after irradiation of multilayer ASW films on TiO2(110). In particular bridge-bonded hydroxyls are formed, but they do not contribute to the H2 produced near the ASW/ TiO2(110) interface. Instead, the results suggest that the H2 is produced from a stable precursor that is trapped on the TiO2(110) or in the water near the substrate. The proposed mechanism for the H2 production near the ASW/TiO2(110) interface is supported by a kinetic model that semiquantitatively reproduces the main features of the nonthermal reactions. II. Experimental Section The experiments were performed in an ultrahigh-vacuum (UHV) system that has been described previously.13,16 The system consisted of a low-energy electron gun, a closed-cycle helium cryostat, a molecular beamline for adsorbate deposition, an Auger electron spectrometer, and a quadrupole mass spectrometer (QMS). The typical base pressure for the system was 1 × 10-10 torr. The 10 × 10 × 1 mm3 rutile TiO2(110) crystals (CrysTec GmbH or Princeton Scientific) were mounted on a resistively heated tantalum base plate. For temperature monitoring and control, a K-type thermocouple was spot-welded to the base plate. The TiO2 sample was prepared by sputtering with 2 keV Ne+ ions and then annealed for between 2 and 10 min in vacuum at 950 K. Multiple ion sputtering/annealing cycles resulted in a reduced TiO2 sample.5 The oxygen vacancy concentration on the ion sputtered and annealed surface is estimated to be 0.08 ( 0.01 MLTi on the basis of the magnitude of the hightemperature OH recombination peak during water temperature programmed desorption (TPD).6 Thin water films were deposited with a molecular beam (flux ≈ 2 × 1014 molecules/cm2 · s) at normal incidence to the surface. During ESD experiments, the electron beam was incident at 35° with respect to the sample normal. Typical instantaneous electron fluxes in the electron beam were ∼0.5-2 × 1015 electrons/cm2 · s with a beam diameter of ∼1.5 mm. The water films were uniformly irradiated by repeatedly scanning the electron beam over a square area that was slightly larger than the film. Each individual scan of the sample was typically 1 s in duration and delivered a fluence of ∼4-16 × 1013 e-/cm2 (depending on the flux). The results presented did not depend on the electron flux over the range specified above (i.e., ESD yields divided by the incident current were a function of the electron fluencesnot the electron flux.) For all the experiments on TiO2(110), the incident electron energy was 100 eV and the ASW films were deposited and irradiated at 100 K. For the experiments on ASW films on Pt(111), the incident electron energy was 87 eV. III. Results A. ESD of Multilayer ASW Films on TiO2(110): Coverage-Dependent H2, O2, and H2O ESD Yields. Figure 1 shows the H2 ESD yields versus electron fluence, φe, for various H2O coverages on TiO2(110). The H2 ESD is complicated and has two kinetically distinct components that we will call “prompt”

Figure 1. H2 ESD signal versus electron fluence for various ASW films on TiO2(110). The coverage of the ASW films (in ML) is labeled on each curve. (a) ASW coverages from 2 to 20 ML. (b) ASW coverages from 20 to 60 ML. (c) Comparison of H2 ESD from 20 and 60 ML ASW films on Pt(111) and TiO2(110).

and “dose-dependent”. For θ > 1 ML, the H2 ESD increases promptly (i.e., with a time characteristic of the experimental response time for the system) when the electron beam is turned on. Subsequently, the H2 ESD signal increases at a dosedependent rate. The relative contribution of the prompt and dosedependent components in the total H2 ESD yield depends on the ASW coverage and electron fluence. At θ ) 20 ML, the maximum H2 ESD yield is approximately two times larger than the prompt H2 ESD yield. For θ > 40 ML, only the prompt H2 component is observed (Figure 1b). For θ e 20 ML in Figure 1, the H2 ESD decreases for larger electron fluences because electron-stimulated reactions reduce the total thickness of the water film, and the H2 ESD yield is smaller for thinner films (see Figure 2a). For comparison with the electron-stimulated reactions on Pt(111), we show the H2 ESD yields for 20 and 60 ML H2O film on Pt(111) in Figure 1c (green and black lines, respectively). The H2 ESD yields versus fluence (i.e., the kinetics) are similar for both TiO2(110) and Pt(111). Note in particular that the H2 ESD from 20 ML ASW on Pt(111) also has two kinetically distinct components, a “prompt” component and a “dosedependent” component,16 and that the yields of these two components are similar on both substrates. This result indicates that, for the same water coverages, the overall reaction rates for producing H2 are similar on both TiO2(110) and Pt(111). However, as discussed below, the particular reactions leading to the “dose-dependent” H2 do depend on the substrate. From data such as that shown in Figure 1, the integrated H2 ESD yield versus water coverage can be calculated (Figure 2a, black circles). We have also investigated the H2O and O2 ESD as a function of ASW coverage (Figure 2a, blue squares and orange triangles, respectively). In each case, the integrated yields are calculated by integrating the corresponding QMS signals versus electron fluence (i.e., versus time where time has been converted to fluence using the measured electron flux). For the

Nonthermal Water Splitting on Rutile TiO2

Figure 2. (a) Integrated H2, O2, and H2O ESD yields (normalized) versus ASW coverage. The integrated ESD yields were calculated for φe ) 8.6 × 1015 e-/cm2. (b) Integrated D2 ESD yields versus ASW coverage for layered films of D2O and H2O on TiO2(110) (black circles) and Pt(111) (red triangles). The results demonstrate that energy absorbed in the H2O portion of the film initiates reactions in the D2O layer adsorbed on the substrate. For both TiO2(110) and Pt(111) substrates, the films were first irradiated with an electron fluence of 4.2 × 1015 e-/cm2 in order to achieve a steady state coverage of precursors on the substrate. The integrated D2 ESD yields were then calculated for a fluence of 2.1 × 1015 e-/cm2.

data in Figure 2, the integration interval was 0 e φe e 8.6 × 1015 e-/cm2 (see Figure 1), and the integrated curves were normalized by their maximum signals. Note that the data are plotted versus the water coverage prior to the electron irradiation. However, the coverage continuously decreases during irradiation due to electron-stimulated sputtering.17 The integrated H2O ESD yield (Figure 2a, blue squares) increases monotonically with coverage up to ∼30 ML and is approximately constant thereafter. The integrated ESD yields for H2 and O2 (Figure 2a, black circles and orange triangles, respectively) versus coverage are very similar. For both H2 and O2, the yields are very small for θ < 2 ML, then reach maxima near 20 ML. Above ∼20 ML, the integrated H2 and O2 yields decrease and reach a coverage-independent level above ∼50 ML. For ASW on Pt(111), the H2 and O2 ESD yields versus coverage also have prominent maxima at θ ≈ 25 ML that are related to reactions at or near the ASW/Pt interface.16,17 As discussed below, the maxima in these yields on TiO2(110) are also related to the reactions near the ASW/TiO2 interface. B. Electron-Stimulated Production of Molecular Hydrogen at the ASW/TiO2 and ASW/Vacuum Interfaces. Previous experiments on Pt(111) showed that the electron-stimulated reactions to produce molecular hydrogen occur preferentially at both the ASW/Pt and ASW/vacuum interfaces.15,16 Thus, the similarity of the H2 ESD yields versus water coverage on TiO2(110) (Figure 2a) and Pt(111)16,17 suggest that the reactions in the ASW films on TiO2(110) are also localized near the two interfaces. To test this hypothesis, we have used layered films of H2O and D2O on TiO2(110) to spatially profile the electronstimulated production of molecular hydrogen within the water films. Figure 3a shows the D2 ESD versus electron fluence for a 20 ML H2O/D2O film where the H2O was deposited on the TiO2(110) and the ratio of H2O to D2O within the 20 ML film was varied. For this experiment, the prompt D2 component (i.e.,

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Figure 3. D2 ESD versus electron fluence, φe, from layered ASW films of H2O and D2O on (a) TiO2(110) for θtot ) 20 ML, (b) TiO2(110) for θtot ) 30 ML, and (c) Pt(111) for θtot ) 30 ML. On each substrate, the D2 ESD was measured for films for increasing coverages of the H2O spacer layer, θSL, while keeping the total ASW coverage, θtot ()H2O + D2O), constant.

for φe < ∼5 × 1014 e-/cm2), which is due to reactions at the vacuum interface, is independent of the H2O space layer coverage, since the H2O is located at the ASW/TiO2 interface. At the same time, the dose-dependent D2 ESD component is suppressed as the H2O coverage increases. Hence, the dosedependent D2 component is related to D2 molecules produced near the ASW/TiO2 interface. Panels b and c of Figure 3 show similar experiments for 30 ML H2O/D2O films deposited on TiO2(110) and Pt(111), respectively. For ASW on platinum, there is a distinct induction period during which no molecular hydrogen is produced at the ASW/ Pt interface (Figure 3c). In contrast, the dose-dependent component of molecular hydrogen produced near the TiO2(110) (Figure 3a and b) does not have any definite induction period comparable to that observed on Pt(111). The induction period is associated with the accumulation of atomic hydrogen on the Pt(111) until a saturation coverage is obtained. Once the Pt(111) is saturated with atomic hydrogen, the H atoms begin to react to form H2.16,18 Below, we will show that the dose-dependent H2 ESD on TiO2(110) is also associated with hydrogenation of the substrate. To test for hydrogen production in the “bulk” of the ASW films, H2O-D2O-H2O “sandwich” films were deposited on TiO2(110) with a 2.4 ML D2O layer grown at various positions within the H2O film (total H2O coverage ) 28.4 ML). Figure 4 shows the integrated D2 ESD from the D2O layer versus the H2O “spacer layer” coverage, θSL, where the “spacer layer” is defined as the layer deposited directly on the TiO2. For these experiments, the D2 ESD yields versus electron fluence (which are similar to the results shown in Figure 3) were integrated over two different fluence intervals: (1) The “prompt” D2 ESD corresponds to the first φe ≈ 2 × 1014 e-/cm2 of the irradiation.

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Figure 4. Integrated D2 ESD versus H2O spacer layer coverage, θSL. The integrated D2 ESD from 2.4 ML of D2O adsorbed at different locations within 28 ML H2O films was measured. The prompt D2 ESD (red diamonds)scorresponding to φe ≈ 2 × 1014 e-/cm2sis only appreciable when the D2O layer is near the top of the film. The total D2 ESD (black circles)scorresponding to φe ≈ 1 × 1016 e-/cm2shas maxima when the D2O layer is on top and when it is adsorbed on the TiO2(110).

Figure 5. Integrated D2 ESD versus H2O spacer layer coverage, θSL for composite H2O and D2O films on TiO2(110) (open and filled circles) and Pt(111) (triangles). The integrated D2 ESD yields were calculated for φe ) 7.8 × 1015 e-/cm2 and 8.0 × 1015 e-/cm2 for the experiments on TiO2(110) and Pt(111), respectively. On TiO2(110), the D2 ESD yield decreases more quickly when the surface is hydrogenated prior to depositing the water film (filled circles).

(2) The “total” D2 ESD yield is the signal integrated over an electron fluence of ∼1 × 1016 e-/cm2. The “prompt” D2 ESD signal is the largest when the D2O layer is deposited at the ASW/ vacuum interface (Figure 4, red diamonds),19 and drops rapidly when the D2O layer is located deeper within the film. The total integrated D2 ESD yield is the largest when the D2O layer is deposited near the ASW/TiO2 or ASW/vacuum interfaces (Figure 4, black circles). In contrast, relatively little D2 is produced when the D2O is deposited in the middle of the H2O film. These results show that molecular hydrogen is created preferentially at the ASW/vacuum and ASW/TiO2(110) interfaces. However, control experiments suggest that rapid mixing of the films during the electron irradiation, which brings D2O and HDO to the interfaces, is responsible for some of the D2 ESD observed when the D2O layer is (initially) in the middle of the H2O film. This electron-stimulated mixing, which we have previously investigated on Pt(111),17 is also responsible for the apparently larger attenuation length for D2 produced at the vacuum interface as seen in the total H2 yield compared to the prompt H2 yield. Figure 5 shows the integrated D2 ESD yields versus the coverage of an H2O spacer layer (θSL) for ASW films on TiO2(110) (open circles) and Pt(111) (triangles) for the experiments shown in Figure 3b and c. The D2 ESD yields decrease

Petrik and Kimmel exponentially as the H2O coverage increases on both substrates. For TiO2(110) the 1/e constant is apparently ∼6.0 ML, while for Pt(111) it is ∼0.9 ML. However, the electron-stimulated mixing of the initially layered films, coupled with the dosedependence of the hydrogen produced near the TiO2(110) increases the 1/e constant measured in this experiment.20 This effect will be discussed in more detail, along with the results of an experiment designed to reduce the effect (Figure 5 solid circles), in Section IV.A. In another experiment, we deposited a two-layer H2O-D2O system, where the H2O layer is deposited on top of the D2O layer (data not shown). For this configuration, the dosedependent D2 yield from the ASW/TiO2 interface remained unchanged, and the prompt D2 yield was suppressed, decreasing exponentially with 1/e constant of ∼2.7 ice ML. Essentially identical results were obtained for ASW on Pt(111).16 Hence, the prompt D2 component is related to D2 molecules produced near the ASW/Vacuum interface, and it is not affected by the substrate. The isotopic layering experiments (e.g., Figures 3-5) demonstrate that molecular hydrogen is produced preferentially at or near the two interfaces. Another experiment with layered H2O and D2O films demonstrates that energy transfer from the “bulk” of the water film to the ASW/TiO2(110) interface is important for the hydrogen produced at that interface. Figure 2b (black circles) shows the integrated D2 ESD from 3 ML of D2O deposited on TiO2(110) versus the total water coverage (i.e., D2O plus H2O) as H2O is deposited on top of the D2O layer. (For θ e 3 ML, the data show the D2 ESD yields from neat D2O versus D2O coverage.) The D2 ESD yield from 3 ML D2O without any H2O on top is relatively small, but it increases with increasing H2O coVerage by an order of magnitude, maximizes at ∼20 ML, and then decreases to a very low level for θ > 50 ML. Figure 2b provides direct evidence for the transfer of energy, but not hydrogen atoms or hydrogen-containing molecules, from the H2O portion of the film to the D2O/TiO2 interface. This energy transfer initiates reactions near the buried ASW/TiO2(110) interface leading to molecular hydrogen production. Energy transfer to the ASW/substrate interface is also important in the electron-stimulated reactions near the ASW/ Pt(111) interface.15,16 For example, Figure 2b (red triangles) shows the integrated D2 ESD yield from 2 ML of D2O on Pt(111) capped with H2O versus the total water coverage. For comparable electron fluences, the D2 ESD yield versus H2O cap layer coverage is similar on both substrates. Several factors contribute to the maxima in the ESD yields shown in Figure 2. First, the ESD yields initially increase as the ASW film thickness increases because more of the electrons are being stopped in the water, thus producing more of the electronic excitations that initiate the reactions. The yields increase until essentially all the energy from the incident electrons is absorbed in the water. Second, as shown in Figure 2b, the distance over which energy can be efficiently transferred through the ASW is limited. Therefore, for sufficiently thick films, the energy from the electron beam is deposited near the ASW/vacuum interface, and few reactions occur at the ASW/ substrate interface because of the limited transfer distance. Finally, since the reactions at the ASW/substrate interface appear to have a higher yield (e.g., compare the “prompt” and “dosedependent” D2 ESD yields in Figure 3), the decrease in the reactions near the substrate for thicker films leads to maxima in the total ESD yields. However, since electron-induced sputtering of the ASW films is significant17 and the H2 and O2 ESD yields are dose- and coverage-dependent, the apparent

Nonthermal Water Splitting on Rutile TiO2

Figure 6. H2O TPD spectra. (a) 20 ML ASW films were irradiated at 100 K, and then heated to 400 K to desorb the remaining water. About 0.5 ML H2O was then redeposited and the TPD spectra were obtained for electron fluences of 0, 2, 4, 6, and 8 × 1014 e-/cm2. Inset: H216O TPD spectra for 20 ML H218O films that were irradiated (red line) and not irradiated (black line). (b) Bare TiO2(110) was irradiated to increase the vacancy concentration prior to depositing ∼0.5 ML H2O.

coverage of the maximum yield is not directly related to the coverage at which the reactions at the ASW/substrate interface are the most efficient, or to the “penetration depth” of the incident electrons. The results in Figures 2 and 3 demonstrate that molecular hydrogen, once created near the buried ASW/TiO2(110) interface, diffuses out through the film without significant isotopic exchange and desorbs as a kinetically distinct, dose-dependent component. At 100 K and coverages less than ∼50 ML, H2 molecules diffuse through the ASW films relatively quickly,16 and the ESD signal drops quickly after the electron beam is turned off (see Figure 1). The prompt decrease at the end of the irradiation shows that the slowly increasing ESD yield at the beginning (i.e., the dose-dependent component) is not due to time needed for hydrogen to diffuse through the film. C. Hydrogen Production near ASW/TiO2 Interface and Hydrogenation of TiO2(110) Surface. The dose-dependent kinetics for the molecular hydrogen produced near the ASW/ TiO2 interface (Figures 1 and 3a) suggest that more than one reaction is required to produce H2. We have performed several experiments to gain further insight into the number of reactions involved, possible reaction intermediates, and the role of the TiO2 in the reactions. Our previous experiments on the nonthermal reactions in ASW films adsorbed on Pt(111) showed that hydrogenation of the substrate is directly related to the production of H2 at the water/Pt interface,16,18 and the results presented below also indicate that hydrogenation of TiO2(110) during electron irradiation of adsorbed ASW films is related to the H2 production in this system. Figure 6a shows a series of H2O TPD spectra. For these experiments, 20 ML H2O films were irradiated with electron fluences from 0 to 8 × 1014 e-/cm2 at 100 K. The films were then annealed to 400 K to desorb the remaining H2O before 0.5 ML was redosed and the TPD spectra obtained. (By first desorbing the irradiated water multilayer and then redosing a controlled amount of H2O, changes in the desorption profile of the water directly adsorbed on the substrate versus the electron

J. Phys. Chem. C, Vol. 113, No. 11, 2009 4455 fluence can be more readily distinguished.) For the unirradiated water film, the TPD spectrum has a peak at ∼300 K due to molecular water desorbing from Ti4+ sites, and a small peak at ∼550 K due to recombinative desorption of 2 bridging hydroxyls.6,9,21 For the irradiated water films, two changes are observed in the water TPD spectra as the electron fluence increases: First, the magnitude of the OH recombination peak increases and shifts to lower temperature. Second, the peak in the TPD spectra due to water desorbing from the Ti4+ rows shifts to lower temperatures and the peak narrows. Note that the maximum electron fluence in this experiment is about 10% of the maximum fluences used in Figure 3, so these results illustrate the changes in the initial stages of the irradiation. Changes in the water TPD spectra similar to those in Figure 6a are observed when electron irradiation of bare TiO2(110) is used to increase the coverage of bridging hydroxyls, [OHb].13 Electron irradiation of bare TiO2(110), results in the desorption of atomic oxygen and thus increases the bridging oxygen vacancy concentration,13,22,23 and since water readily dissociates in these vacancies to produce 2 OHb per vacancy, [OHb] also increases. Figure 6b shows a series of 0.5 ML H2O TPD spectra after irradiation of the bare TiO2(110) surface with various electron fluences. The similarities between the water TPD spectra in Figure 6a and b indicate that the concentration of OHb on the TiO2(110) increases during the initial stages of irradiation of a multilayer water film with energetic electrons. However, for the irradiated water films (Figure 6a), the changes in the TPD spectra (i.e., the increase in the OH recombination peak and the shift of the TPD ∼275 K peak) are more pronounced suggesting that higher coverages of OHb are produced. For larger electron fluences than those shown in Figure 6, there are significant changes in the region of the TPD spectra associated with water desorbing from the Ti4+ sites that are difficult to interpret (data not shown). The peak in the water TPD spectra at ∼500 K is due to bridging hydroxyls reacting to form water and a bridging oxygen vacancy: 2OHb f H2O + Ob + Ov.6,9,21 Therefore, if hydrogen atoms are added to bridging oxygen atoms during the irradiation of thick water films, some of the water that desorbs at ∼500 K should contain oxygen atoms from the substrate. This can be tested by an experiment where the oxygen in the water films is isotopically labeled. The inset to Figure 6a shows an experiment where a 20 ML H218O film on Ti16O2(110) was irradiated, and then the H216O (red line) (and H218O, data not shown) desorbing in the OHb recombination peak were monitored. For comparison, the H216O recombination peak is also shown for a 20 ML H218O film that was not irradiated (inset of Figure 6a, black line). The observation that the H216O recombination peak in the TPD spectra increases in magnitude and occurs at lower temperature after irradiation also indicates that bridging oxygen atoms were hydroxylated during the irradiation of the 20 ML water film. The formation of bridging hydroxyls during irradiation is related to the dose-dependent kinetics observed for the H2 produced near the ASW/TiO2(110) interface. Figure 7a shows the H2 ESD yield versus electron fluence for a 20 ML H2O film deposited on TiO2(110) and irradiated with 100 eV electrons at 100 K (solid black line). The surface was then annealed at 190 K for 60 s to desorb the multilayer water film while leaving water adsorbed at the Ti4+ sites and the bridging hydroxyls. This surface was then dosed with another 20 ML at 100 K and irradiated a second time at 100 K (Figure 7a, solid blue line). For the second irradiation, the initial H2 yield is the same as for a 20 ML film irradiated the first time. However, the H2 yield increases more quickly for the second irradiation, while saturat-

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Petrik and Kimmel deposited and the film was irradiated a second time while monitoring the H2, D2, and HD ESD yields. The key observations are that (1) the D2 and HD ESD yields are very small (Figure 7b, blue and green lines, respectively) and (2) the H2 ESD yield versus time (Figure 7b, red line) is enhanced compared to the H2 ESD yield for a 20 ML film on sputtered and annealed TiO2(110) (Figure 7b, black line). Moreover, it does not matter what isotope (D2O or H2O) was irradiated during the first cycle: the second H2 ESD kinetics are the same (compare purple and red lines in Figure 7b). These results indicate that the OHb do not directly contribute to the electroninduced H2 production. However, in those cases where the concentration of OHb is increased (by the first irradiation), the H2 ESD kinetics are accelerated compared to kinetics for a surface with a lower initial OHb concentration. These results show that the hydroxylation of the TiO2(110) surface does influence the kinetics, and the model presented below also supports this conclusion.

Figure 7. H2, HD, and D2 ESD versus electron fluence for 20 ML ASW films (solid lines). A Monte Carlo model (dashed lines) reproduces the H2 ESD from several experiments. (a) The solid black line shows the H2 ESD for an ASW film on annealed TiO2(110). After waiting 60 s at 100 K, this film was irradiated a second time (solid red line). The solid blue line shows the H2 ESD from a 20 ML ASW film on a hydrogenated surface. (b) When 20 ML D2O films are irradiated, desorbed and then 20 ML H2O is redeposited, the D2 (green) and HD (blue) ESD signals are small during the second irradiation. However, the H2 ESD (red) is the same as when a 20 ML H2O film is irradiated both times (purple) and accelerated compared to when a 20 ML H2O film is irradiated the first time (black).

ing at the same value for longer irradiation times. If the same experiment is repeated with an annealing temperature of 300 K, then the H2 ESD for the second irradiation (Figure 7a, solid purple line) also increases more quickly than the first irradiation but not as quickly as when the film was only annealed to 190 K. For annealing temperatures above ∼600 K, the H2 ESD kinetics revert to those characteristic of an ASW film deposited on a sputtered and annealed surface. Note that 400-600 K corresponds to the temperature range where oxygen vacancies are regenerated via recombinative desorption of bridging hydroxyls (see Figure 6).6,13,21 Figure 7a also shows that the H2 ESD for a 20 ML ASW film that was first irradiated with a fluence of ∼7.8 × 1015 e-/ cm2, then irradiated for a second time after a delay of 60 s at 100 K (solid red line). For the second irradiation, the H2 ESD yield quickly reaches approximately the level achieved at the end of the first irradiation cycle (and then decreases at longer times due to electron-stimulated sputtering). If the ASW film is briefly annealed at temperatures up to 150 K between the first and second irradiation, then the H2 ESD signal also promptly returns to approximately the level it had at the end of the first irradiation showing that the precursors for the H2 ESD are stable for at least several minutes at these temperatures. The results in Figure 7 suggest that desorbing the irradiated multilayer water films removes some, but not all, of the species that are responsible for the dose-dependent H2 kinetics. To test if the hydrogen in the OHb is incorporated into the H2 produced near the ASW/TiO2 interface, we again utilize experiments with H2O and D2O (Figure 7b). For these experiments, a 20 ML D2O film was irradiated to increase the coverage of ODb on the substrate. Then, the remaining D2O was desorbed by heating the film to 300 K. Next, a 20 ML H2O film was

IV. Discussion A. Electron-Stimulated Reactions in ASW Films. Some of the main observations from the results presented above are (1) H2 is preferentially made at or near the ASW/vacuum and ASW/TiO2 interfaces. (2) The H2 made near the vacuum interface appears promptly and is independent of the electron fluence, while the H2 produced near the TiO2(110) is initially zero and increases to a steady-state level with increasing electron fluence. (3) For H2 made near the ASW/TiO2 interface, most of the energy necessary for the reactions is absorbed in the overlying water layers. However, atomic diffusion of hydrogen atoms (or molecular diffusion of hydrogen-containing species) from the region where the excitations are produced to the interface is not primarily responsible for the reactions, instead mobile excitations (such as H2O* or H3O+) are involved (Figure 2b). (4) Irradiation of ASW films results in hydrogenation of the TiO2(110), initially through the formation of bridging hydroxyls (Figure 6). (5) The hydrogen atoms in the bridging hydroxyls are not incorporated into the H2 made near the ASW/ TiO2 interface (Figure 7). Next, we discuss a model of the electron-stimulated reactions that can qualitatively account for these observations. The electron-stimulated reactions begin with the absorption of the energy of the incident electron in the ASW film, primarily by ionization and electronic excitation of the water molecules.24-27 Following the initial excitations, several processes can occur very rapidly.27,28 For example, geminate electron-ion recombination can produce electronically excited water molecules (H2O+ + e- f H2O*) and hydronium ions can be created by fast proton transfer reactions between water ions and surrounding water molecules (H2O+ + H2O f H3O+ + OH). Electrons can also be captured by species other than H2O+ (e.g., OH + e- f OHand H3O+ + e- f H3O*), or trapped in the ice e- f e-trap. For sufficiently thin films, the energetic electrons will also produce excitations in the substrate which could subsequently initiate reactions in the adsorbed water films. However, we have yet to find any compelling evidence that excitations in the substrate are a significant factor in the nonthermal reactions in ASW films on TiO2(110). For example, even though the distribution and lifetimes of electronic excitations in TiO2(110) and Pt(111) should be very different, the nonthermal reactions in ASW films adsorbed on these two surfaces are quite similar (see Section IV.C). The same reasoning suggests that secondary electrons from the substrate are not a dominant source for the observed reactions on these two substrates. Trapped or solvated

Nonthermal Water Splitting on Rutile TiO2 electrons in the ASW films also do not appear to play a major role in the reactions at the ASW/TiO2(110) interface. For example, in experiments where we irradiated ASW films with electrons whose energies were below the threshold for ionizations and excitations in the ASW (i.e., ∼5 ML) leads to reduction of the TiO2(110) via the formation of OHb (and also probably HTi). In contrast, we have recently shown that irradiation of water films for θ < 1.5 ML (i.e.,