Origin of Coverage Dependence in Photoreactivity of Carboxylate on

Sep 30, 2015 - The influence of reactant coverage on photochemical activity was explored using scanning tunneling microscopy (STM) and ultraviolet pho...
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Origin of Coverage Dependence in Photoreactivity of Carboxylate on TiO2(110): Hindering by Charged Coadsorbed Hydroxyls Zhi-Tao Wang,† Michael A. Henderson,‡ and Igor Lyubinetsky*,† †

EMSL, Institute for Integrated Catalysis, and Pacific Northwest National Laboratory, Richland, Washington 99352, United States Fundamental and Computational Sciences Directorate, Institute for Integrated Catalysis, and Pacific Northwest National Laboratory, Richland, Washington 99352, United States

‡

S Supporting Information *

ABSTRACT: The influence of reactant coverage on photochemical activity was explored using scanning tunneling microscopy (STM) and ultraviolet photoelectron spectroscopy (UPS). We observed diminished reactivity of carboxylate species (trimethyl acetate, TMA) on TiO2(110) as a function of increasing coverage. This effect was not linked to intermolecular interactions of TMA but to the accumulation of the coadsorbed bridging hydroxyls (HOb) deposited during (thermal) dissociative adsorption of the parent, trimethylacetic acid (TMAA). Confirmation of the hindering influence of HOb groups was obtained by the observation that HOb species originated from H2O dissociation at O-vacancy sites have a similar hindering effect on TMA photochemistry. Though HOb’s are photoinactive on TiO2(110) under ultrahigh vacuum conditions, UPS results show that these sites trap photoexcited electrons, which in turn likely (electrostatically) attract and neutralize photoexcited holes, thus suppressing the hole-mediated photoreactivity of TMA. This negative influence of surface hydroxyls on hole-mediated photochemistry is likely a major factor in other anaerobic photochemical processes on reducible oxide surfaces. KEYWORDS: scanning tunneling microscopy, ultraviolet photoelectron spectroscopy, trimethyl acetate, hydroxyl, TiO2

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coverage-dependent effects makes it difficult to identify the major contributors.3,10 One of the convenient probe molecules for photocatalytic studies on TiO2(110) is trimethylacetic acid (TMAA).14,17−20 Similar to other carboxylic acids, TMAA adsorbs dissociatively at ≥150 K via deprotonation (heterolytically), with the acid proton forming an adjacent hydroxyl group and the carboxylate species, trimethyl acetate (TMA), bridge-bonding across two terminal Ti sites.21,22 Besides, a second adsorption channel exists at VO-sites, in which one O atom of TMA fills a vacancy and the other O bonds to a neighboring Ti site.23 The TMA species adsorbed at regular Ti sites exhibits relatively straightforward hole-mediated photolysis under ultraviolet (UV) exposure, producing CO2 and a tert-butyl radical, the latter of which undergoes rapid thermal reactions yielding only gaseous products.24,25 In contrast, we have recently demonstrated that TMA photoreactivity at VO sites is completely inhibited.26 Sole previous study of coverage effects on TMA photolysis was conducted on an oxidized TiO2(110) surface and under aerobic conditions.14 Coverage dependence in the absence of the oxygen, in its function as an electron scavenger, has not been explored. It should be also noted that the

hotocatalytic reactions on TiO2-based materials continue to attract much attention due to potential applications ranging from water splitting to environmental remediation of organic pollutants.1,2 Among several structural aspects critically affecting the photoreactivity on TiO2, an influence of reactant coverage is relatively unexplored.3,4 A number of potential coverage-dependent factors have been proposed on the basis of a few photochemical studies of organic5−9 and nonorganic reactants,10−12 over a diverse list of surfaces, for example, metals,5,7 metal oxides,6,8,9,12 alkali halides,11 and elementary semiconductors.10 Specifically, a sharp decrease in the photoactivity, often observed at high coverage, was attributed to either quenching of the excited species due to intermolecular interactions,5,11 or to separate reaction mechanisms operating over the low-to-high coverage range.9,10 Other possible aspects considered include work function and/or adsorbate conformational changes,7 as well as site blocking.6,8,12 On the model photocatalyst surface, such as the rutile TiO2(110), coverage dependence has not been extensively examined.13−15 For O2 photodissociation at the O-vacancy (VO) sites on TiO2(110), in particular, we have found no dependence on the coverage.15,16 On the other hand, a strong decay for acetone photooxidation on TiO2(110) at saturation coverages was observed, likely caused by the inability of gasphase O2 to interact with an organic-covered surface.13 In general, the likelihood that multiple factors contribute to © XXXX American Chemical Society

Received: August 18, 2015 Revised: September 24, 2015

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ACS Catalysis

not photoreactive,3,19 they become visible to STM after the adjacent TMA/Ti5c’s are depleted. On the other hand, the amount of TMA/VO species remains virtually unchanged because TMA photoreactivity at VO sites is completely suppressed,26 as mentioned earlier. We have examined the kinetics of TMA/Ti5c photodepletion for different initial concentrations of TMA by determining the dependence of the TMA/Ti5c coverage, ⊖(TMA/Ti5c), as a function of UV irradiation time, as shown in Figure 2a.

potential influence of coadsorbed hydroxyl species has not been considered in the literature. In this Letter, we report a considerable decrease in the anaerobic photoactivity of TMA over TiO2(110) with increasing coverage, and we show that it is caused by a buildup of coadsorbed hydroxyls that concurrently trap excited electrons, suppressing the hole-mediated photochemistry of TMA. Figure 1 shows STM images illustrating TMAA adsorption on a partially reduced TiO2(110) surface, along with changes

Figure 1. STM images (a) before and (b) after adsorption of 0.12 ML of TMAA on TiO2(110) at 300 K, and following UV irradiation for (c) 15 s and (d) 90 s (at 250 K). Concentration of TMA/Ti5c’s decreases from 0.08 ML in (b) to 0.04 ML in (c) and 0.008 ML in (d), while TMA/VO density remains constant at 0.06 ML.

Figure 2. (a) Normalized TMA/Ti5c concentration as a function of the UV irradiation time for several initial coverages. Inset shows the plots in a semilog scale; the lines are linear regressions, the slopes of which correspond to reaction rate constants. (b) Dependence of photodepletion rate constant on the initial TMA/Ti5c coverage (the line is a guide to the eye).

resulting from UV light irradiation.27 The empty-state STM image of clean TiO2(110) in Figure 1a displays alternating bright and dark rows of 5-fold coordinated Ti (Ti5c) and 2-fold coordinated bridging O (Ob) atoms, respectively. There is also a low density (∼0.07 ML) of bridging oxygen vacancies (VO).28 The presence of several bridging hydroxyl (HOb) groups is due to the minor background water dissociation at VO sites. Upon dissociative adsorption of TMAA, TMA species form at both Ti5c (termed TMA/Ti5c) and VO (TMA/VO) sites, as discussed above.21−23 The individual TMA/Ti5c or TMA/VO species, seen as isolated bright spots, can be identified by the distinct location, centered either on Ti5c or Ob rows, respectively, as indicated in Figure 1b. In addition, TMA/VO species often appear “fuzzy” in comparison with the more rounded and slightly larger TMA/Ti5c species.23 For both adsorption sites, the resulting HOb groups are essentially undetectable by STM while residing alongside the adsorbed TMA’s.22 In order to follow the kinetics of TMA photochemistry, we acquired STM images after progressive, stepwise irradiation of the sample with UV light. Selected STM images after two successive UV irradiation periods are shown in Figures 1c,d. These show a gradual decrease in the TMA/Ti5c coverage due to the photoinduced decomposition and production of volatile products,24,25 as discussed above. Because the HOb groups are

TMMA/Ti5c species were counted in STM images after illuminating the same surface in a step-by-step fashion, while TMA/VO’s were excluded from the statistics. Between 300 and 500 TMA/Ti5c species have been analyzed at each initial coverage, for 10 coverages overall. In each case, the amount of TMA/Ti5c species quasi-exponentially decays to zero (see inset of Figure 2a), indicating close to first-order kinetics. (Note that this is not always the case for photoreactions on TiO2(110). For instance, the photodesorption of molecular oxygen cannot be described by a simple exponential function.15) However, the data revealed a rather strong, initial coverage dependence in the reaction rate constant for TMA/Ti5c photodepletion, as summarized in Figure 2b. Evidently, the rate becomes slower for higher initial coverages, reaching a quasi-saturation value for ⊖(TMA/Ti5c) > 0.2 ML, whereas the variance in the rate from very low coverage to ∼0.4 ML is approximately a factor of 4. A number of potential factors could be responsible for the initial coverage dependence. A change in the optical response of TiO2(110) can be excluded as we do not expected this to be sensitive to interfacial, submonolayer TMA coverage. Fur6464

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ACS Catalysis thermore, simple carboxylates are not known to strongly absorb light in the UV spectral region. It also seems unlikely that two separate photochemical mechanisms operate at the extremes of the coverage scale (high and low), since by all available data, the TMA is consistently bridging across two Ti5c sites irrespective of coverage. In the next section, we explore the potential role of coadsorbed surface hydroxyl, the other product of dissociative TMAA adsorption, on TMA photochemistry. In order to examine the possible influence of HOb groups on TMA photochemistry, the initial surface coverage of HOb was adjusted using preadsorption of water. H2O spontaneously dissociates at VO site (above ∼180 K) to form two HOb species.28 Figure 3 shows STM images after exposure of

Figure 4. (a) Dependence of the rate constant of TMA photodepletion (red) on the initial ⊖(TMA/Ti5c) for TiO2(110) with predosed 0.05 ML H2O (⊖(HOb) ∼ 0.1 ML); the plot for the case with no predosed H2O (blue) is reproduced from Figure 2b). (b) Dependence of the photodepletion rate constant on the initial ⊖(HOb) for different amounts of predosed H2O; the same plot (blue) from (a) is replotted (black) in HOb-coordinate (taking into account the HOb’s associated with both TMA/Ti5c and TMA/VO species).

varies with the TMA/Ti5c initial coverage, but the values are noticeably less than ones obtained without predosed water (blue line), leading to the conclusion that addition of (extra) HOb groups causes a substantial decrease of the rate of TMA/ Ti5c photolysis. On the other hand, the dependencies of TMA/Ti 5c photodepletion in the two cases match when plotted as a function of the initial HOb coverage, whether it is from TMAA dissociation alone or along with dosed water (Figure 4b). (Data for the different amounts of predosed H2O also fit the same plot.) All this indicates that the HOb species, regardless of its source, inhibits TMA photochemistry. The pronounced dependence of TMA on an initial coverage then essentially arises from the accumulation of HOb species and not, for example, TMA−TMA interactions, as was previously suspected.14 The underlying inhibition mechanism of HOb was explored with UPS (Figure 5a). After the consecutive adsorption of H2O and TMAA, the He I spectrum (black) exhibits a largely unperturbed valence band of TiO2(110).20 The weak Ti 3dderived peak at ∼0.8 eV in the band-gap region, shown in detail in Figure 5b, is associated with reduced Ti3+ cations related to VO defect sites.29,30 (The major peaks derived from HOb and TMA species, are located at higher binding energies outside the valence band, and can be seen in He II spectra shown in Figure S3.) It should be noted that upon dissociation of TMAA (and accompanying formation of HOb species), the original Ti 3d peak does not increase, as shown in Figure S4, indicating the absence of electron donation to the TiO2,20 and consisting with adsorbed H species being protons (which do not have an electron to donate). Upon increasing UV exposure, the Ti 3d peak noticeably grows (Figure 5b), which can be assigned to preferential trapping of the photoexcited electrons as surface Ti3+-HOb species.31−33 The photoexcited holes, on the other hand, initiate the decomposition of TMA/Ti5c’s. However, we propose that the accumulation of trapped electrons at HOb sites hinders TMA photochemistry. We have previously shown that this is the case for TMA groups adsorbed at VO sites.26 Specifically, it is possible that trapped electrons electrostatically attract and recombine with the photoexcited holes, effectively decreasing the number of holes that could reach TMA/Ti5c species, and thus suppress photochemistry. On a separate note,

Figure 3. STM images after (a) TiO2(110) exposed to H2O (⊖(HOb) ∼ 0.13 ML) and (b) subsequent adsorption of 0.17 ML of TMAA at 300 K, and following UV irradiation for (c) 15 s and (d) 90 s (at 250 K). Concentration of TMA/Ti5c’s decreases to 0.14 ML in (c) and 0.03 ML in (d).

TiO2(110) to H2O, followed by TMAA adsorption and subsequent UV irradiations. Figure 3a demonstrates that the dissociative adsorption of H2O (at 300 K) hydroxylates almost all of the VO sites. After subsequent TMAA exposure to the hydroxylated surface (Figure 3b), only TMA/Ti5c species are detected, but the prior HOb species, as well as the new formed ones, are obscured (and virtually no TMA/VO species were observed). The general characteristics of TMA photoreactivity are not changed by hydroxylation of the VO sites, with UV irradiation gradually depleting the surface TMA/Ti5c groups (Figure 3c) and the HOb’s once again visible to STM (Figure 3d). (Prolonged UV irradiation eventually removes nearly all TMA/Ti5c species, leaving behind only the HOb groups, as illustrated in Figure S1, Supporting Information.) However, the TMA/Ti5c photodepletion rate constant was significantly affected by surface prehydroxylation. The rate constants for several initial TMA/Ti5c coverages (determined in the same way as above, see Figure S2) with and without predosed H2O are shown in Figure 4a. The photodepletion rate constant for the case with preadsorbed HOb (red dots) also 6465

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Figure 6. Dependence of the rate constant for electron trap charging (pink) on the coverage of HOb species. The plot for TMA photodepletion rate constant (black) is reproduced from Figure 4b.

function of the HOb coverage, which demonstrates that the process is slower for higher coverages. While we believe that this is one of the first observations that a rate constant for trap charging depends on the coverage, the underlying mechanism is not clear. On the other hand, the coverage dependence for the rate constant of the trapping is found to reasonably match the analogous plot for the TMA photodepletion (black) in Figure 6, while both processes follow the first-order kinetics. However, it is not entirely unexpected that the process of hole-mediated TMA photodepletion and of photoexcited electron trapping should have similar rate constants for the same HOb coverage, because it is generally assumed that for photochemistry to be effective processes enabled by different types of charge carriers should occur at balanced rates.3 Finally, we point toward another interesting aspect. Looking back at Figure 4a, the substantial decrease of the reaction rate constant after VO hydroxylation (using H2O) suggests that the excess electrons associated with OHb species hinder TMA photochemistry more efficiently than the original VO sites. Recently, Yates and co-workers have proposed that the dissociative adsorption of water at VO’s redistributes (“spreads out”) the spatial extent of the unpaired excess electrons, based on their observation of a decreasing O+ yield in electron stimulated desorption.35 While it is generally accepted that the excess electrons associated with VO defects are delocalized toward nearest subsurface Ti atoms,28 it is conceivable that VO hydroxylation may alter the nature of this localization.35,36 Theoretical calculations, however, offer a conflicting picture,36−38 with some results indicating that excess electrons at OHb’s are delocalized similarly to those at VO sites.37,38 In conclusion, employing STM and UPS, we have observed a strong nonlinear decay of the rate constant with coverage for the photolysis of trimethyl acetate on TiO2(110). This inhibition stems from a buildup of the coadsorbed hydroxyl species, taking into account that upon (thermal) dissociative adsorption of trimethylacetic acid, the equal amount of TMA and OHb species is generated concurrently. Confirmation of the inhibiting influence of HOb groups was obtained by dissociating H2O at O-vacancy sites, resulting in an additional hindering effect on TMA photochemistry. While hydroxyl species are not highly photoreactive, they are not merely spectators, functioning as trap sites for photoexcited electrons. We postulate that these trapped electrons attract and neutralize photoexcited holes, adversely affecting the hole-mediated photochemistry of TMA. In this sense, the photochemical rate dependence on

Figure 5. (a) UPS He I spectra of TiO2(110) after consecutive adsorption of 0.05 ML H2O and 0.08 ML TMAA, and following incremental UV light exposures. (b) Close-up view of Ti 3d region of the spectra. (c) Area of the Ti 3d peak, normalized to the saturation level, as a function of the UV irradiation time; the line is an exponential fit. (Offset of Ti 3d intensity is proportional to the VO defect density.)

UPS did not reveal any substantial bend bending during the photochemical reaction, which could be induced by the surface hydroxylation of the semiconductive TiO2(110),34 and thus affect the rate of holes approaching the surface. We also evaluated the accumulation of charge at the HOb traps by measuring the intensity of Ti 3d peak as a function of the UV irradiation time, as shown in Figure 5c. It is wellrecognized that the HOb groups originated from H2O dissociation retain the unpaired extra electrons associated with VO’s.29 In contrast, nominally neutral HOb species originated from the TMAA (thermal) deprotonation become charged only upon trapping of the photoexcited electrons.20,32 The latter process is indeed responsible for gradual increase of Ti 3d peak seen in Figure 5c. This signal eventually saturates, presumably indicating the charging of every available HOb trap (nominally, at the level of one electron per trap, as we have shown previously).20 The kinetic profile for this reasonably fits an exponential dependence, indicating a first-order process. Note that it takes about 100 s to charge nearly all traps for ∼0.08 ML of HOb species (Figure 5c). Because a certain level of UV irradiation is required for charging all HOb species stemmed from TMAA deprotonation, their full impact on TMA photochemistry would be somewhat delayed. In contrast, HOb groups generated upon H2O dissociation at VO sites are already charged as a result of the inherent properties of those sites. This difference likely accounts for the somewhat smaller rate constants (i.e., greater inhibition) of TMA photodepletion with versus without predosed H2O, as seen in Figure 4b. We also found that the process of photochemical charging of the HOb electron traps depends on the coverage of HOb. Figure 6 shows the corresponding rate constant (pink) as a 6466

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(16) Henderson, M. A.; Shen, M.; Wang, Z.-T.; Lyubinetsky, I. J. Phys. Chem. C 2013, 117, 5774−5784. (17) Ohsawa, T.; Lyubinetsky, I. V.; Henderson, M. A.; Chambers, S. A. J. Phys. Chem. C 2008, 112, 20050−20056. (18) Ohsawa, T.; Lyubinetsky, I.; Du, Y.; Henderson, M. A.; Shutthanandan, V.; Chambers, S. A. Phys. Rev. B: Condens. Matter Mater. Phys. 2009, 79, 085401. (19) Du, Y.; Petrik, N. G.; Deskins, N. A.; Wang, Z.; Henderson, M. A.; Kimmel, G. A.; Lyubinetsky, I. Phys. Chem. Chem. Phys. 2012, 14, 3066−3074. (20) Wang, Z.-T.; Garcia, J. C.; Deskins, N. A.; Lyubinetsky, I. Phys. Rev. B: Condens. Matter Mater. Phys. 2015, 92, 081402. (21) White, J. M.; Szanyi, J.; Henderson, M. A. J. Phys. Chem. B 2004, 108, 3592−3602. (22) Lyubinetsky, I.; Yu, Z. Q.; Henderson, M. A. J. Phys. Chem. C 2007, 111, 4342−6. (23) Lyubinetsky, I.; Deskins, N. A.; Du, Y.; Vestergaard, E. K.; Kim, D. J.; Dupuis, M. Phys. Chem. Chem. Phys. 2010, 12, 5986−5992. (24) Henderson, M. A.; Epling, W. S.; Peden, C. H. F.; Perkins, C. L. J. Phys. Chem. B 2003, 107, 534−45. (25) White, J. M.; Henderson, M. A. J. Phys. Chem. B 2005, 109, 12417−12430. (26) Wang, Z.-T.; Deskins, N. A.; Henderson, M. A.; Lyubinetsky, I. Phys. Rev. Lett. 2012, 109, 266103. (27) See Supporting Information for experimental details. (28) Dohnalek, Z.; Lyubinetsky, I.; Rousseau, R. Prog. Surf. Sci. 2010, 85, 161−205. (29) Kurtz, R. L.; Stockbauer, R.; Msdey, T. E.; Roman, E.; de Segovia, J. L. Surf. Sci. 1989, 218, 178−200. (30) Zhang, Z.; Jeng, S.-P.; Henrich, V. E. Phys. Rev. B: Condens. Matter Mater. Phys. 1991, 43, 12004−12011. (31) Szczepankiewicz, S. H.; Colussi, A. J.; Hoffmann, M. R. J. Phys. Chem. B 2000, 104, 9842−9850. (32) Henderson, M. A.; White, J. M.; Uetsuka, H.; Onishi, H. J. Am. Chem. Soc. 2003, 125, 14974−5. (33) Di Valentin, C.; Pacchioni, G.; Selloni, A. Phys. Rev. Lett. 2006, 97, 166803−4. (34) Zhang, Z.; Yates, J. T. Chem. Rev. 2012, 112, 5520−5551. (35) Zhang, Z.; Cao, K.; Yates, J. T. J. Phys. Chem. Lett. 2013, 4, 674− 679. (36) Liu, L. M.; McAllister, B.; Ye, H. Q.; Hu, P. J. Am. Chem. Soc. 2006, 128, 4017−4022. (37) Deskins, N. A.; Rousseau, R.; Dupuis, M. J. Phys. Chem. C 2009, 113, 14583−14586. (38) Minato, T.; Sainoo, Y.; Kim, Y.; Kato, H. S.; Aika, K.-i.; Kawai, M.; Zhao, J.; Petek, H.; Huang, T.; He, W.; Wang, B.; Wang, Z.; Zhao, Y.; Yang, J.; Hou, J. G. J. Chem. Phys. 2009, 130, 124502−11.

TMA coverage correlates with the affinity of HOb species to trap negative charge. The rate constants for the hole-mediated TMA depletion and electron trapping both depend similarly on HOb coverage, follow the first-order kinetics, and have comparable absolute values, as anticipated for the photoprocesses enabled by opposite charge carriers. Overall, this work provides new fundamental insights into the roles of surface defects and coadsorbates on oxide photochemistry. Because hydroxyl formation is a common phenomenon resulting from dissociative adsorption of numerous species (water, carboxylic acids, alcohols, etc.), a similar negative influence on hole-mediated photochemistry should be expected on other reducible oxides.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.5b01819. Experimental details, STM image after prolonged UV exposure, kinetic profile for H2O pre-exposed surface, entire He II UPS spectra, and Ti 3d region of the He I UPS spectra (PDF)

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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 supported by the US Department of Energy (DOE), Office of Basic Energy Sciences, Division of Chemical Sciences, and performed at Environmental Molecular Sciences Laboratory (EMSL), a national scientific user facility sponsored by the DOE’s Office of Biological and Environmental Research and located at PNNL.

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REFERENCES

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DOI: 10.1021/acscatal.5b01819 ACS Catal. 2015, 5, 6463−6467