Photostability and Thermal Decomposition of Benzoic Acid on TiO2

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Photo-Stability and Thermal Decomposition of Benzoic Acid on TiO Elizabeth C Landis, Stephen C Jensen, Katherine R Phillips, and Cynthia M. Friend J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 11 Sep 2012 Downloaded from http://pubs.acs.org on September 11, 2012

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The Journal of Physical Chemistry

Photo-stability and thermal decomposition of benzoic acid on TiO2 Elizabeth C. Landis1, Stephen C. Jensen1, Katherine R. Phillips1, Cynthia M. Friend1,2* 1. Department of Chemistry and Chemical Biology, Harvard University, 12 Oxford St, Cambridge, MA 02138, United States 2. School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts 02138, United States AUTHOR EMAIL ADDRESS [email protected] RECEIVED DATE (to be automatically inserted after your manuscript is accepted if required according to the journal that you are submitting your paper to)

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ABSTRACT. Carboxylic acid moieties are used to anchor organic dyes to TiO2 in many applications; 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

therefore, their structure, distribution on the surface, and thermal and photochemical stability are extremely important.

Herein, we investigate the thermal and photochemistry of benzoic acid

monolayers on TiO2. Benzoic acid deprotonates to form benzoate monomers on the TiO2 surface, and thermally decomposes to benzene and CO. Titanium interstitials migrate to the surface and react with oxygen atoms from the benzoate molecules to form new TiOx islands.

A combination of X-ray

photoelectron spectroscopy and scanning tunneling microscopy show that the benzoate layer on TiO2 is stable under UV illumination for several hours. The stability of benzoate under illumination is a contrast to the photochemical instability of other small carboxylic acid containing molecules on TiO2 and demonstrates the importance of substituents on the stability of the molecular layer.

KEYWORDS: rutile TiO2(110), photochemistry, interstitial, temperature programmed reaction, scanning probe microscopy


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Introduction 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The thermal and photochemistry of carboxylic acids adsorbed on TiO2 is important because carboxylate functional groups are used to tether dye sensitizers to titania in solar cells.1 Carboxylic acids are also components of atmospheric pollutants that can be degraded on titania.2 For dye systems, stable binding between the carboxylic acid and TiO2 is essential. Key properties including absorption, valence electronic structure, thermal stability, and photochemical stability depend on the functionality of the carboxylic acid group. Formic acid, acetic acid, and trimethyl acetic acid all deprotonate and bind to rutile TiO2 (110) as carboxylates, but their decomposition mechanisms differ.3-5 Acetic acid and trimethyl acetic acid both decompose under UV illumination in aerobic conditions; acetic acid yielding mostly ketene, and trimethyl acetic producing a mixture of isobutene and isobutane.4,6-9 Adsorbed oxygen is thought to increase decomposition by scavenging excess electrons created from excitons during illumination thus allowing holes to further decompose the adsorbed layer.7,8 Benzoic acid is known to form well-ordered monolayers on TiO2 (110) in which the carboxylic acid groups deprotonate to benzoate, which binds to five-fold coordinated titanium ions.10,11 Scanning tunneling microscopy (STM) images of the molecules were interpreted as dimers that form due to interactions between the phenyl rings of adjacent molecules; however, we provide evidence in this paper that leads to a reinterpretation of the data.11-13 X-ray absorption spectroscopy and angle resolved X-ray photoelectron spectroscopy indicate that the ring is approximately perpendicular to the surface, in agreement with our interpretation.10,12 In this letter we describe the thermal and photochemical behavior of benzoate formed from benzoic acid on bulk reduced rutile TiO2(110) as a means of better understanding the role of the hydrocarbon group in the behavior of photosensitizers containing carboxylates. We find that benzoate is remarkably resistant to decomposition upon UV illumination in the presence of adsorbed oxygen. We discuss this in the context of the electronic structure of the phenyl moiety compared to simple akyl groups.


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Experimental Methods 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

TiO2(110) crystals were purchased from SurfaceNet; samples were prepared and routinely cleaned by Ar+ sputtering (1-2 kV, ~2 µA, 20-30 min) followed by annealing to 850-1000 K. This treatment was sufficient to eliminate impurities measured by X-ray photoelectron spectroscopy (XPS) and produce flat terraces for scanning tunneling microscopy (STM) experiments.

Repeated

sputter/anneal cycles lead to a bulk reduced crystal that had ~6% surface site defects as measured by the desorption of water, a sharp (1×1) low energy electron diffraction pattern and counting bridging oxygen vacancies density in STM images. An electrospray deposition system from Molecular Spray (UHV4) was used to dose benzoic acid (C7H6O2, Alfa Aesar, 99.5+%) solutions in acetone (Alfa Aesar, 99.5+%) onto the TiO2 surfaces. Depositions are performed at room temperature and pressures between 5x10-6 and 5x10-7 Torr. The spot size of the electrospray system is approximately 4 mm in diameter. Since this size is smaller than the samples used for XPS and thermal reaction spectroscopy, the sample is translated along the electrospray beam during the spray period. The spray was monitored using in-situ mass spectroscopy to confirm that the parent ion m/z 122 was present. The deposition time is varied, with a 0.2 M solution and a 5 minute spray used for STM experiments. Thermal reaction spectra and XPS data are collected after a 10 minute spray of 0.1 M solution unless otherwise noted. Electrospray deposition of benzoic acid dissolved in acetone on TiO2(110) at room temperature resulted in a surface with both benzoic acid and acetone adsorbed. The surface was warmed to 340 K, which desorbed acetone leaving benzoic acid behind. In a separate control, acetone was electrosprayed on the reduced surface and no additional features were observed in STM images after warming to 340K. Isotope labeled benzoic acid was C6D5COOH (Cambridge Isotope Laboratories, 98%).

In situ mass spectrometry during deposition showed no

istotope switching with the acetone solvent. STM experiments were performed on an Omicron variable temperature microscope that has been described previously.14 Temperature programmed reaction experiments were carried out in a chamber


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that is also equipped with a Perkin-Elmer 04-548 dual-anode X-ray source with a SPECS EA-10 Plus 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

hemispherical energy analyzer, Phi model 11-020 low energy electron diffraction optics (LEED), and a Hiden HAL RC 511 quadrupole mass spectrometer used to determine thermal reaction products. A preparation chamber is equipped with a Physical Electronics 20-045 Ar+ gun for sputtering the surface and the electrospray deposition system from Molecular Spray (UHV4). Thermal reaction spectra were collected using a heating ramp of 2.5 K/s while recording the intensity for several product masses.

The selectivities were calculated using a method described

previously15,16 in which the peak intensities for each fragment were integrated over time and corrected for the ionization efficiencies of different fragments, mass discrimination of the quadrupole filter, fragmentation, and the electron multiplier gain. X-ray photoelectron (XP) spectra are collected at a pass energy of 36 eV with a 0.0610 eV step size. Carbon spectra are collected for 100 scans, titanium and oxygen are collected for 10. The peaks are fit using Casa XPS software using a Shirley background correction and Lorentzian peaks. UV-illumination was performed with a 300 W Xe Arc Lamp. A 400 nm cut-on filter (3.1) eV was used to match the band gap of TiO2 and reduce crystal warming during illumination. With the filter in place, the surface was illuminated with 92 mV/cm2, with 14% of the incoming light below the TiO2 bandgap. Density Functional Theory calculations were carried out using plane wave code implemented by the Vienna Ab initio Simulation Package. We used PAW pseudopotentials and the GGA-PW91 functional commonly used for other calculations on the TiO2(110) surface. Additional details for the calculations can be found in the supporting information.

Results and Discussion Benzoate thermally decomposes on reduced TiO2 (110), yielding gaseous CO and benzene as the primary products at 700 K (Fig 1). A minor amount of C6H5CO is also formed. A stoichiometric ACS Paragon Plus Environment

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amount of oxygen is deposited in both of these reative processes. No benzoic acid or biphenyl is 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

detected in these experiments. No other products were detected in a comprehensive search between m/z 12 and m/z 215. Benzene accounts for 44±5% of the total products, carbon monoxide and CO2 are 46±4% and 0.9±-.2%, respectively. A fragment with m/z 105 composes 8±3% of the products. We identify this as the fragment C6H5CO, although we cannot rule out a fragment of larger products such as phenyl benzoate. We do not detect m/z 106 to indicate the production of benzaldehyde. The m/z 78:77 ratio of 3:1 confirmed that benzene, not phenyl, is evolved into the gas phase. Thus, the main pathway is decomposition of benzoate to form stoichiometric amounts of benzene and CO, leaving behind an oxygen atom on the surface. A likely source for the additional hydrogen required for benzene formation is the competing decomposition of the phenyl ring that accompanies CO2 formation. This assertion is supported by the observation of carbon deposition after thermal decomposition of benzoate using X-ray photoelectron spectroscopy, described below. We use d5-labeled benzoic acid to study the formation of benezene and fate of dissociated protons. Isotope labeling studies with d5-benzoic acid result in a 3.4:1 ratio of m/z 84: m/z 83, attributed to d6-benzene production and d5-benzene, respectively. These assignments are confirmed by the ratios of m/z 83:81 and m/z 84:82:80, which are consistent with the expected spectrum of isotope labeled benzene (see supporting information Figure S1) These data demonstrate that the primary source of hydrogen for benzene formation is ring decomposition, but 23% of the protons do come from other sources, likely including dissociated protons.

We also observe hydroxyl group

recombination as water evolution at 480 K, which is another possible fate of dissociated protons, although we cannot differentiate from surface hydroxyl groups formed during the electrospray. The rate of benzene evolution from benzoic acid decomposition is limited by reaction, since benzene itself desorbs from rutile TiO2 (110) at ~300 K.17 The temperatures for product evolution and the relative mass quantities do not change at lower benzoate coverages (see Supporting Information Figure S2).


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Defects including oxygen vacancies18 and titanium interstitials19,20 are known to drive reduction 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

of other oxygenates, e.g. benzaldehyde, on TiO2 (110) and most likely also play a role in benzoate reduction to benzene and CO. Dosing water on the reduced TiO2 surface to heal surface oxygen vacancies and leave a hydroxylated surface does not affect the benzoate thermal reaction products (Fig S4). The hydroxyl groups recombine at 480K, which produces gaseous water and bridging oxygen vacancies;21 hence, vacancies are present during the benzoate decomposition. Reduced TiO2 (110) can also be oxidized through O2 dosing to heal the oxygen vacancies. Adsorbed oxygen molecules are also known to quench titanium interstitials, which migrate to the reduced TiO2 surface at temperatures as low as 400 K.19,22 Postdosing oxygen after benzoic acid electrospray deposition and heating to desorb acetone does not change the thermal reaction products, suggesting that oxygen is not able to displace the strongly-bound benzoate molecules (shown in Supporting Information Figure S5).

Quenching of

interstitials prior to dosing benzoic acid is not possible because O adatoms react with acetone to form strongly-bond complexes that desorb at 400 K rather than 330 K observed for acetone on the reduced surface. Oxygen adatoms also lead to acetone decomposition of undetermined mechanism.23 Our imaging studies show that titanium interstitials are active in the benzoate thermal reaction (see below). Thermal reduction of benzoate leads to the formation of TiOx islands on the surface, accounting for the oxygen lost during reduction of benzoic acid. (Figure 2) Islands appear in STM images obtained after thermal decomposition of benzoic acid by heating the surface to 850 K. The new features are 1.23 ± 0.12 nm across the Ti rows by 1.56 ± 0.19 nm along the rows and 0.31 ± 0.02 nm high. The height agrees well with a calibrated TiO2 step height of 0.32 nm. We attribute the new features to new TiOx islands formed as titanium intersitials visit the surface so as to drive the reductive processes. One oxygen is lost from the benzoate during production of both benzene and CO as well as benzaldheyde. The degree of order and size of the TiOx islands depends on the time the sample is maintained at 850 K. The formation of new TiOx islands from adsorbed oxygen atoms in benzoic acid is similar to the observations that adsorbed formate molecules interact with titanium interstitials to form new TiO2


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islands.24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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The features we observe are similar in dimension to those created by O2 dosing, and

following benzaldehyde coupling to form stilbene.19,20,25 X-ray photoelectron spectroscopy (XPS) provides supporting evidence that benzoate is formed and remains intact up to 500 K in our experiments. There are two C(1s) peaks at 285.5 eV and 289.5 eV in a ratio of ~6:1. The peak at 285.5 eV is attributed to the phenyl ring, whereas that at 289.5 eV is assigned to the carbon in the carboxylate group (Fig 3b), based both on the 6:1 ratio and on previous reports of these relative binding energies reported for benzoate on TiO2.10 The absolute binding energies differ from previous reports due to different O(1s) reference binding energies. After benzoate thermal reaction, 4% of the carbon remains on the surface, which is consistent with the temperature programmed reaction results indicating that decomposition occurs. The Ti(2p) and O(1s) spectra match the expected oxidation states of TiO2 and are attenuated with benzoic acid electrospray deposition with no change in their oxidation state (Figure S6). We use scanning tunneling microscopy to identify the configuration of benzoate on the TiO2 surface. The benzoate features in Figure 4a are 1.14 ± 0.10 nm along the Ti rows by 1.00 ± 0.13 nm across the rows.13,14 Asymmetric dimensions of benzoate on TiO2 have previously been attributed to benzoate dimers,11 but we attribute these features to single benzoate molecules. Our recent studies of butyrophenone photochemistry on TiO2, which yields benzoate, demonstrate a 1:1 ratio between adsorbed butyrophenone molecules and features due to the benzoate photochemistry product.26 Additionally, the feature length is consistent with a single benzoate molecule. Density functional theory calculations provide an estimate of the benzoate bond energy of 2.2 eV (Figure 4b) for the most stable geometry—a bridging configuration in which the benzoate oxygen atoms are bound to adjacent titanium atoms and 1.26 eV for the bidentate configuration in which the benzoate oxygen atoms are bound to the same titanium atom with the phenyl ring oriented along the rows (configurations shown in Figure S7). The bridging configuration was previously calculated to be the most stable configuration for benzoate on rutile TiO2.27 Binding of the benzoate in bridging oxygen ACS Paragon Plus Environment

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vacancies, which has been observed for formate,28 failed to converge in DFT runs and was not observed 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

in our STM images. This is presumably due to steric effects with the phenyl ring and the bridging oxygen row. There was no measureable photodecomposition of benzoate bound to TiO2(110) under conditions where there is a substantial degree of photoreduction of trimethyl acetate, based on X-ray photoelectron spectra collected between a series of UV illuminations (Figure 4c, d). The stability was tested under aerobic conditions because oxygen on the TiO2 surface is thought to scavenge excess electrons from excitons formed during illumination and, thus, promote hole driven processes that are proposed to remove π-electron density from the carboxylate portion of molecules and lead to elimination of the carboxylate group.7-9 The total amount of carbon and peak area ratios on the surface remained constant following the oxygen dose and each subsequent illumination. There was likewise no change the reaction products, their yields, or the temperatures of their evolution subsequent to exposure to UV light (Figure S7). Scanning tunneling microscopy images likewise show that there is little change in the benzoate layer on TiO2 through a series of UV illuminations (Figure 5). Illumination with UV light for 2.25 hours with an oxygen dose, shown in Figure 5b, did not significantly change the amount or orientation of benzoate molecules on the TiO2 surface. The stability of benzoate in the presence of O2 and UV light is a contrast to trimethyl acetate. Trimethyl acetate, which has the same carboxylate functionality and which is known to degrade after as little as 15 minutes exposure to UV light in the presence of a 10-7 torr background of O2,6,9 is clearly depleted in our experiments (Figure 5c-d). In our experiments, trimethyl acetate degraded in 35 minutes under UV illumination, when using a lamp 9 times more powerful than previous experiments.29 Our studies in conjunction with those in the literature demonstrate that molecular structure plays a key role in both thermal and photochemical stability of carboxylate moieties on TiO2. Previous work has shown that protection against β-hydrogen dehydrogenation leads to increasing thermal stability in trimethyl acetic acid molecular layers compared to other carboxylic acids such as formic acid.18,30 The ACS Paragon Plus Environment

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presence of a tertiary carbon atom in the β position of benzoate and the resonance stabilization of the 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

phenyl group is consistent with its high thermal stability, which is comparable to trimethyl acetic acid, which thermally reacts primarily at 660 K.30 In the case of benzoate, thermal reduction at 700 K is driven by reduced Ti—most likely interstitials—that form TiOx islands in conjunction with the loss of CO and benzene. Although the thermal stability of these two carboxylates is similar, the photochemical stability of benzoate on TiO2 is much greater than trimethyl acetate. This difference can be rationalized by considering the absorption mechanism for light above the band gap of TiO2. Incident photons adsorbed by the TiO2 surface create electron hole pairs within 1 µm of the TiO2 surface and the appearance of Ti3+ in the electron energy loss spectrum.31 Holes that do not recombine can react with the adsorbed organic layer to create oxidized, excited carboxylate molecules.9 Molecular orbital calculations of benzoate on copper show that the highest occupied molecular orbitals are located on the phenyl ring.32 The participation of delocalized orbitals may inhibit photodegradation due to resonance stabilization of the hole created on the molecule. Trimethyl acetic acid and acetic acid, both of which degrade under illumination in aerobic conditions, lack the stability of conjugation. Electron donating and withdrawing substituents on aromatic rings affect the hydroxylation products in TiO2 aqueous heterogeneous catalysis.33 Experiments testing the reactivity of ring-substituted benzoic acid could be used to test this effect in vacuum. Thermal and photo-stability are essential in photosensitization of TiO2. In dye-sensitized solar cell applications, substitution and conjugation of the β position of the carboxylate linkers is used to improve light adsorption and electron injection.34-36 Our work shows that the β-position substitution can also impact sensitizer stability and a phenyl group bound to the carboxalate could provide both stability and conjugation. Conclusions We demonstrate that benzoate is thermally and photochemically stable on TiO2. The molecule decomposes at 700K, fragmenting primarily into CO, benzene, and a C5H5CO fragment. The thermal ACS Paragon Plus Environment

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stability of the molecular layer can be attributed to its protection against β-hydrogen elimination due to 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

the tertiary carbon atom. We attribute the photochemical stability to the relative instability of the benzene radical photolysis product.

ACKNOWLEDGMENT The authors gratefully acknowledge support funding from NSF grants DMR-0934480 and CHE-0956653. EL thanks the Henson Fund at the Harvard University Center for the Environment. SJ thanks the NSF for a graduate research fellowship. The authors thank Robert J. Madix for insightful comments.

SUPPORTING INFORMATION PARAGRAPH Experimental details for temperature programmed reaction spectra quantification and supplemental spectra, XPS data collection and peak fitting, DFT calculations. This material is available free of charge via the internet at http://pubs.acs.org.

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Figure Captions Figure 1: Thermal decomposition of benzoate formed from benzoic acid commences at ~600 K, yielding mainly CO (m/z 28) and benzene (m/z 78). Minor amounts of benzaldehyde (m/z 105), and CO2 (m/z 44) are also detected. Benzoic acid (m/z 122) does not desorb intact and no other products are observed. Benzoic acid dissolved in acetone was electrosprayed onto TiO2 at room temperature. Prior


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to thermal reaction, the sample was heated to 500 K to desorb the acetone solvent, which desorbs 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

molecularly at 330K, and water (shown in Figure S3). Figure 2. The growth of TiOx islands following benzoate thermal reduction is demonstrated using STM. (A) Annealing to 850 K without benzoic acid results in a clean surface. A representative bridging oxygen vacancy is indicated with black box. (B) Decomposition of benzoate on reduced TiO2(110) followed by annealing to 850 K for 75 sec. results in small TiOx nanoparticles due to reaction of Ti interstitials with oxygen deposited during the decomposition reaction. The larger images in (A) and (B) are 19.9 x 19.9 nm2. The inset shows a single TiOx nanoparticle and its position with respect to the fivefold coordinate Ti rows represented here with vertial white lines. (3.4 x 2.8 nm2) (The tunneling conditions are the same for all images: Vs= +1.4 V, i = 0.1 nA, T= 300 K). Figure 3. X-ray photoelectron spectra of the C(1s) region provides evidence that benzoate is resistant to photochemical reduction. Spectra show are for: (A) As-prepared, clean TiO2(110); (B) following electrospray deposition of benzoic acid in acetone at room temperature followed by heating to 500 K to desorb the acetone solvent, leaving the benzoate intermediate; (C) After illumination of the benzoate layer, prepared as in (B), with UV light for 15 minutes; (D) subsequent exposure of (C) to 100L of O2 followed by an additional 60 min of UV illumination; and, (E) residual carbon after heating to 825 K to desorb benzoate. Peaks in blue at 285.5±0.1 eV are assigned to the phenyl ring, peaks in green at 289.5±0.1 eV are assigned to the carbonyl carbon, (peak positions and integrations in Table S1, Supporting Information)]. Figure 4. (A) Scanning Tunneling Microscopy (STM) image of benzoate on r-TiO2(110) show Lshaped features aligned along the five fold coordinated Ti rows (denoted by horizantal black lines). These features are attributed to a single benzoate molecule. The scale bar is 0.5 nm. (T=300 K,Vs=2.2 V i= 70 pA ). Benzoate is formed by electrospray deposition of benzoic acid dissolved in acetone at room temperature, followed by heating to 360 K to desob acetone. (B) Molecular model of the most stable configuration of benzoate on TiO2(110), based on density functional theory (DFT) calculations in ACS Paragon Plus Environment

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which both oxygen atoms bind to titanium atoms and the phenyl ring is perpendicular to the surface. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Gray balls denote five-fold coordinated Ti atoms, dark red are in plane oxygen atoms, bright red atoms are bridging oxygen atoms, and pink indicate the oxygen atoms in the benzoate. The scale bar in (B) is also 0.5 nm. Calculations neglect van der Waal forces, which would likely add to the stabilization. Figure 5. Scanning Tunneling Microscopy (STM) shows that benzoate is resistant to photodegradation while trimethyl acetate readily decomposes under illumination with UV light at room temperature. (A) STM image obtained for benzoate on reduced TiO2(110); (B) images of the same area of the benzoate layer in (A) following illumination with UV light for 2.25 hours with saturation coverage O2 dose; (C) trimethyl acetate (the smaller, close-packed features, circled). The larger brighter features are surface contaminants that were present on the as prepared surface. (not shown) (D) Trimethyl acetate decomposes after 35 minutes of irradiation without a saturation coverage of O2. Contaminants are still present after illumination. Benzoate and trimethyl acetate are formed by electrospray deposition of benzoic acid (0.1 M) and trimethyl acetic acid (1.0 M) in acetone at 300 K for 5-10 min and subsequent heating to 360 K to desorb acetone.

Images A and B were taken with Vs=+1.5 V, i=0.15 nA for

tunneling parameters and C and D used Vs=+1.45 V, i=0.24 nA. All images are 15.4x15.4 nm2, the scale bars are 3 nm. Figure 1


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Figure 2


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Figure 3

Figure 4 ACS Paragon Plus Environment

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Figure 5

TOC GRAPHIC: ACS Paragon Plus Environment

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 ACS Paragon Plus Environment

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Photostability and Thermal Decomposition of Benzoic Acid on TiO2

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