Importance of Diffusion in Methanol Photochemistry on TiO2(110

Nov 8, 2012 - The photoactivity of methanol on the rutile TiO2(110) surface is shown to depend on the ability of methanol to diffuse on the surface an...
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Importance of Diffusion in Methanol Photochemistry on TiO2(110) Mingmin Shen,* Danda P. Acharya, Zdenek Dohnálek, and Michael A. Henderson Chemical and Materials Sciences Division, Pacific Northwest National Laboratory, P.O. Box 999, MS K8-87 Richland, Washington 99352, United States S Supporting Information *

ABSTRACT: The photoactivity of methanol on the rutile TiO2(110) surface is shown to depend on the ability of methanol to diffuse on the surface and find sites active for its thermal dissociation to methoxy and hydroxy species. Temperature programmed desorption (TPD) results show that the extent of methanol photodecomposition to formaldehyde is negligible on the clean TiO2(110) surface at 100 K due to a scarcity of sites that can convert (photoinactive) methanol to (photoactive) methoxy. The extent of photoactivity at 100 K significantly increases when methanol is coadsorbed with oxygen, however only those molecules able to adsorb near (next to) a coadsorbed oxygen species are active. Preannealing coadsorbed methanol and oxygen to above 200 K prior to UV irradiation results in a significant increase in photoactivity. Scanning tunneling microscopy (STM) images clearly show that the advent of increased photoactivity in TPD correlates with the onset of methanol diffusion along the surface’s Ti4+ rows at ∼200 K. These results demonstrate that optimizing thermal processes (such as diffusion or proton transfer reactions) can be critical to maximizing photocatalytic reactivity on TiO2 surfaces. for exploration of oxide surface properties.25,28−41 Methanol is also commonly employed as a hole-scavenger in photocatalysis studies on TiO2.15,42−47 Many experimental and theoretical studies have examined methanol chemistry on TiO2(110).25,28,30−35,39,41,48 The majority of these studies suggest that the molecular form is more stable on the ideal TiO2(110) surface, although the small energy difference with the dissociative form suggests that transient dissociation is conceivable.31−33,39 Defects (e.g., steps and vacancies) and coadsorbates (e.g., oxygen adatoms) can also induce dissociation.16,25,34,41 Our previous work has shown that there are two key steps involved in CH3OH photodecomposition on TiO2(110).48 The first step (O−H bond cleavage) is thermal and the second step (C−H bond cleavage) is initiated photochemically. In other words, we find that adsorbed methoxy (and not molecular methanol) is the photochemically active form of methanol on TiO2(110). This conclusion suggests that the extent to which a TiO2 photocatalyst can thermally dissociate methanol to methoxy will dictate how effectively methanol can function as a hole scavenger on that photocatalyst. In this work, we show that methanol diffusion to and subsequent dissociation at coadsorbed O adatoms (Oa) are both key steps in forming methoxy for the photodecomposition of this molecule on TiO2(110). While methanol molecules that happen to adsorb adjacent to Oa species can dissociate at temperatures as low as 100 K, thus becoming photoactive, the majority of methanol molecules isolated from dissociation sites

1. INTRODUCTION The diffusion of molecules across surfaces is important in a wide variety of heterogeneous catalytic processes.1−10 For example, catalytic rates are often determined by the activity of a minority of sites, such as kinks, steps, and point defects,1,11−14 that adsorbates and intermediate species must diffuse to in order to react. Additionally, coadsorbates often must diffuse to each other for the catalytic reaction of interest to proceed. In either situation, the overall reactivity strongly depends on the diffusivity of adsorbates on the catalyst surface. Whereas there are ample examples of the importance of diffusion in thermally driven catalytic reactions on surfaces, there are few examples in the literature in the context of heterogeneous photocatalysis. Titanium dioxide has attracted considerable research attention because of its potential importance in a variety of photocatalytic settings, such as water purification, solar fuel cells, water splitting, and self-cleaning films.15−24 The rutile TiO2(110) surface is perhaps the most studied single crystal oxide surface and consequently has become the prototypical semiconducting oxide surface for fundamental studies of many photocatalytic phenomena. There are several recent examples in the literature in which diffusional processes on this surface have provided new insights into reactivity.9,10,24−27 For example, on fully hydroxylated TiO2(110), water is found to mediate the diffusion of surface species such as bridging OH groups (OHb) that would otherwise be stationary, thus catalyzing reactions by bringing reactants together.10 In this study, we show that the diffusion of methanol on TiO2(110) is important to sustain photocatalytic activity of this molecule on TiO2. Methanol is a simple prototype for many organic compounds and represents a suitable molecular probe © 2012 American Chemical Society

Received: October 2, 2012 Revised: November 7, 2012 Published: November 8, 2012 25465

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3. RESULTS AND DISCUSSION We have previously shown that the TiO2(110) surface can be prepared with only methoxy groups (i.e., no molecular methanol) by heating a coadsorbed mixture of excess methanol and preadsorbed oxygen to ∼350 K.48,55 This treatment reacts methanol and oxygen to form methoxy, according to reactions 1 and 2, and also removes the water product and any unreacted molecularly adsorbed methanol. CH3OHa + Oa → CH3Oa + OH t (1)

remain photoinactive until the advent of diffusion permits the requisite dissociation reaction to proceed.

2. EXPERIMENTAL SECTION The UHV chamber used for photochemistry has a base pressure 900 K. Surface cleaning was accomplished through cycles of Ne+ sputtering and annealing. The cleaning cycles produced a crystal with blue color and a surface with an oxygen vacancy population of ∼5% as gauged by water TPD. Routine cleaning was accomplished by annealing the crystal at 850 K for ∼15−30 min in UHV. These annealing treatments did not significantly increase the oxygen vacancy population over the course of the experiments described here. All gas dosing in this chamber utilized a triply differentially pumped molecular beam doser that delivered reagents in a ∼5 mm spot centered on the crystal face. Methanol coverages were determined through knowledge of the exposure needed to saturate the first layer. The first layer saturation coverage of methanol on the clean TiO2(110) surface is about ∼0.70 ML, varying by a few hundredths of a ML depending on the surface oxygen vacancy population.28,38 The TiO2(110) surface with oxygen adatoms was prepared by O2 exposure 300 K.50−52 UV light irradiation was performed using a fiber optic light delivery system coupled to 365 nm UV light-emitting diode (LED) light source (Prizmatix), with the incident light perpendicular to the surface and covering an area larger than the dosing spot area. In this geometry, the flux of UV light from this source was typically about 4 × 1016 photons cm−2 s−1. TPD experiments were performed with a heating rate of 2 K/s and the sample facing the quadrupole mass spectrometer (QMS). The sample temperature was measured using a type K thermocouple directly attached to the sample surface with a ceramic cement (Aremco). The STM experiments were carried out in an ultrahigh vacuum chamber (base pressure ≤1 × 10−10 mbar) equipped with an Omicron variable temperature scanning tunneling microscope (VT-STM). Well-ordered partially reduced rutile TiO2(110) surface was prepared by repeated cycles of Ne+ sputtering and 950 K annealing of the single crystalline TiO2 sample (10 × 2 × 0.5 mm3, Princeton Scientific). The surface order and cleanliness were checked using low energy electron diffraction (LEED) and Auger electron spectroscopy (AES). Electrochemically etched and UHV-annealed tungsten tips were used for imaging. In all experiments, constant current tunneling mode with a positive sample bias voltage was employed. The STM scan rate of ∼0.7 ms/point (∼ 2 min/ frame) was used and the STM images were analyzed using WSxM software (Nanotech).53 The concentration of oxygen vacancy sites on the sample ranged from 7% to 9%. Methanol (Sigma-Aldrich, 99.9%) was dosed on TiO2(110) directly in the STM stage using a retractable tube doser with a 2 μm pinhole aperture.54 The sample temperature was measured by means of a silicon diode pyrometer at the cooling stage, although there was a temperature offset between the cooling stage and the sample. During cooling/heating experiments, the actual sample temperature was about ±15 K off from that specified by the silicon diode.

CH3OHa + OH t → CH3Oa + H 2Oa

(2) 52,56−58

Coadsorbed oxygen thermally dissociates on heating, and the resulting O adatoms (Oa) promote methoxy formation by facilitating CH3O−H bond cleavage. As it turns out, all available Oa species could be utilized for methanol dissociation by employing excess methanol and heating the surface to >300 K. However, the progression of these processes at low coverages and temperatures is unclear. We employed the following experimental scheme to explore the interdependences of coverage and surface temperature on methanol photoactivity. The key steps involved heating the coadsorbed system to different temperatures, followed by UV irradiation at ∼155 K for 5 min and then postirradiation TPD. (A 5 min irradiation period was selected for convenience. Longer irradiation times did not render conflicting results.) Figure 1 presents these TPD

results. The left and right sides of Figure 1 show mass 30 (representing photochemical formaldehyde formation) and 31 (representing consumption of methoxy groups) TPD traces respectively after different treatments of ∼0.1 ML methanol adsorbed at 100 K on an initial ∼0.03 ML Oa covered TiO2(110) (The Oa coverage on TiO2(110) was prepared by

Figure 1. Mass 30 (left) and mass 31 (right) TPD spectra of less than 0.1 ML methanol adsorbed on ∼0.03 ML oxygen adatom preadsorbed TiO2(110), (a) without UV irradiation, (b) after UV irradiation at 155 K, (c) flashed to 200 K, then 5 min UV irradiation at 155 K, (d) flashed to 225 K, then 5 min UV irradiation at 155 K, (e) flashed to 250 K, then 5 min UV irradiation at 155 K. 25466

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(mass 30) are constant, and thus independent of the preheating conditions. Above 200 K, the amounts of these increase with increasing preheating temperature indicative of a higher efficiency of photodecomposition. (For comparison, the points highlighted with dashed circles representing peak areas obtained without UV irradiation.) Small variations of each individual dosing could lead to some coverage difference (within 3% of the measured coverage), which will not influence the trend of diffusion enhanced photochemistry. To obtain evidence for our assertion that methanol diffusion is responsible for enhanced methanol photoactivity above 200 K, we performed STM measurements of methanol on TiO2(110) as a function of sample temperature. At temperatures below 200 K, no methanol diffusion was observed on the surface after dosing, which is consistent with our TPD observations in Figures 1 and 2 based on methoxy photodecomposition results. Figure 3 displays STM images of the

dosing oxygen at 300 K.) TPD with only steps 1 and 2 (i.e., no UV irradiation) is shown in trace a of Figure 1 (black traces). The intense mass 31 peak at ∼630 K was an indication of methoxy formation in the reaction of methanol and preadsorbed oxygen adatom,28,55,59 whereas the weak mass 30 peak at ∼325 K was from QMS cracking of a small amount of excess methanol in the experiment. (As we have shown previously,55 coadsorption of 0.1 ML CH3OH and 0.03 ML Oa should result in ∼0.06 ML CH3O, with the remainder of the adsorbed CH3OH retained as molecular species.) In trace b of Figure 1, 5 min UV irradiation at 155 K (skipping step 3) resulted in some mass 30 TPD peak intensity at ∼300 K due to formaldehyde formation (reaction 1), with the methoxy peak at 630 K decreasing slightly. This limited activity at low coverage suggests that at 100 K some methanol molecules adsorbed near Oa species, possibly via a precursor-mediated adsorption process that allows limited sampling of the surface, and are thermally dissociated to methoxy groups at 155 K. However, that the majority of adsorbed methanol in this low coverage constraint were still present on the surface as the (photoinactive) molecular form. The amount of methoxy photodecomposition did not change when the methanol and Oa coadsorbed mixture was preheated to temperatures between 155 and 200 K (data not shown). However, the amount of methoxy photodecomposition increased notably as the methanol and Oa coadsorbed mixture was preheated to above 200 K prior to photolysis at 155 K (trace c of Figure 1). Additional heating above 200 K showed further increases in the formaldehyde yield (left) and the extent of methoxy consumption (right) in the postirradiation TPD (traces d and e of Figure 1). Normalized mass 30 and 31 TPD peak areas from data in Figure 1 were plotted in Figure 2 as a function of the

Figure 3. STM snapshots of the same area with adsorption of methanol on reduced TiO2(110), methanol dosing at 110 K and scanning at 230 K (bias voltage: +1.2 V, tunneling current: 86 pA). (a) time = 0 min; (b) line profile of three different features in (a); (c) 4 min after (a); (d) 9 min after (a). Yellow circles show original positions of molecular methanol at Ti row as in image (a).

same area obtained after methanol adsorption on TiO2(110) at ∼110 K and STM scanning at 230 K where the onset of methanol diffusion is observed. Part a of Figure 3 represents a typical STM image taken of the surface after methanol adsorption. The bright blue rows are identified as the 5-fold coordinated Ti4+ ions (Ti5c), and the dark rows are the bridgebonded oxygen (Obr) ions.16,25 (The number density of VO sites on the clean TiO2(110) surface was about 7% with respect to the population of Ti5c sites.) The faint blue spots between the Ti5c rows (one of which is marked with a red line) are assigned to bridge-bonded oxygen vacancy (VO) sites. Two methanol-related features were detected on this methanol covered surface. The bright spots between Ti5c rows (one marked with a magenta line) corresponded to dissociative methanol at VO sites.25,35 Additionally, bright spots on the Ti5c rows (one marked with a green line) were due to molecularly

Figure 2. Normalized mass 30 (315 K) and 31 (630 K) peak area (most of them taken from Figure 2) as a function of sample flashing temperature before UV irradiation at 155 K. The points highlighted with dashed circles representing peak areas obtained without UV irradiation.

preheating temperature to further identify the temperature range at which methanol photoactivity is enhanced on TiO2(110). These data show two regions of methanol photoactivity as a function of surface preheating temperature delineated at ∼200 K. Below 200 K, the amounts of methoxy photodecomposition (mass 31) and formaldehyde formation 25467

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adsorbed methanol species.25,35 There was no evidence in the STM for bridging OH groups adjacent to these bright spots, negating their assignment to dissociated methanol. Line profiles of these features along the arrows shown in part b of Figure 3 (i.e., the [001] direction) illustrate the apparent height differences of these three features, with the VO sites appearing at a height of ∼0.35 Å, dissociated methanol at ∼0.9 Å,25 and molecular methanol at ∼1.25 Å.35 Parts c and d of Figure 3 show two STM images taken 4 and 9 min respectively after the one in part a of Figure 3. Yellow circles in these two images show the original positions of molecular methanol species at Ti5c sites in part a of Figure 3. (The yellow star marks a bright contamination spot on the TiO2(110) surface that was used as a reference point.) There were obvious position changes of these methanol species in the images of parts c and d of Figure 3 resulting from diffusion along the Ti5c rows. Some methanol molecules diffused to VO sites and dissociated. (Methoxy groups at VO sites and Oa’s were stationary under these conditions.60,61) These diffusion events were possible at ∼230 K (a movie with a larger area is shown in the Supporting Information), and diffusion was too rapid to track at temperatures above 250 K (data not shown). At 230 K we determine the hopping rate to be 1.15 ± 0.3 s−1. (We have confirmed that this value did not change as we reduced the frame repletion rate clearly showing that methanol diffusion at 230 K is not induced by the STM tip.) The hopping rate value can be further used to estimate the diffusion barrier of 0.68 ± 0.01 eV assuming Arrhenius dependence and the preexponential factor of 1012 s−1. This value should be used with caution since significant deviations from the typical values of preexponential factors were previously observed; for example, for diffusion of H2O on Ti5c rows.62 These findings are consistent with previously published results that indicate methanol diffusion is rapid at room temperature on TiO2(110).24,25,33 However, our low temperature STM results provided evidence for the onset of methanol diffusion as being the reason for enhanced photoactivity seen in TPD at temperatures above ∼200 K. A cartoon of the diffusion and dissociation process is shown in Figure 4. The methanol dissociation event and its temperature dependence is similar to the process of water dissociation at Oa sites on TiO2(110), as reported previously.10 Methanol molecules that adsorb far from Oa species must first diffuse to these species and dissociate to methoxy before the photoreaction can proceed.

4. CONCLUSIONS In summary, molecular methanol diffusion was studied on vacuum annealed rutile TiO2(110) by TPD and STM. Both experimental approaches (surface chemistry and imaging) pointed to the absence of methanol diffusion on TiO2(110) below ∼200 K. We also show that methanol diffusion is key to maximizing the photoactivity of this molecule on the TiO2(110) surface.

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ASSOCIATED CONTENT

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AUTHOR INFORMATION

S Supporting Information *

STM movie of large area (21.4 nm × 21.4 nm, 230 K scanning, bias voltage: +1.2 V, tunneling current: 86 pA) showing diffusion of methanol molecules and dissociation at VO sites. This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the US Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences & Biosciences. Pacific Northwest National Laboratory (PNNL) is a multiprogram national laboratory operated for DOE by Battelle under contract DEAC0576RL01830. The research was performed using EMSL, a national scientific user facility sponsored by the Department of Energy’s Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory.

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Figure 4. Schematic model of methanol diffusion at elevated sample temperature to meet oxygen adatom and form methoxy through dissociation. 25468

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