Adsorption and Photocatalytic Degradation of 3-Fluoroaniline on

Article pubs.acs.org/JPCC

Adsorption and Photocatalytic Degradation of 3‑Fluoroaniline on Anatase TiO2(101): A Photoemission and Near-Edge X‑ray Absorption Fine Structure Study Mark J. Jackman and Andrew G. Thomas* School of Physics and Astronomy and Photon Science Institute, Alan Turing Building, The University of Manchester, Oxford Road, Manchester, M13 9PL, United Kingdom S Supporting Information *

ABSTRACT: Synchrotron radiation photoelectron spectroscopy and near-edge X-ray absorption fine structure (NEXAFS) techniques have been used to study the adsorption of 3-fluoroaniline on an anatase TiO2(101) single crystal. Photoemission results suggest that 3-fluoroaniline bonds through the amine group to a 5-fold coordinated surface Ti atom. Independently, 3-fluoroaniline also bonds to the surface via the fluorine atom; however, the carbon−fluorine bond undergoes fission to leave fluorine bound to the surface. Using the searchlight effect, the carbon K-edge NEXAFS spectra recorded for 3-fluoroaniline on anatase TiO2(101) shows the phenyl ring to be oriented at 90 ± 5° from the surface and twisted 10 ± 5° relative to the ⟨010⟩ direction. When 3-fluoroaniline bonded on anatase TiO2(101) is irradiated with UV light (4.8 eV), the molecule is almost completely desorbed from the surface.

1. INTRODUCTION TiO2 is of significant interest for applications covering a broad range of the technological spectrum. Its use has been investigated for photovoltaics, self-cleaning surfaces, biomaterials, and photocatalysts.1−4 TiO2 is also a promising candidate for a commercial photoelectrode for photoelectrochemical water splitting. TiO2 is extremely resistive to corrosion and photocorrosion in aqueous environments; its properties can be tuned via control of defect chemistry and doping;5,6 it is an abundant material and therefore has a low cost; and it is nontoxic to both humans and the environment. Unfortunately for applications requiring absorption of sunlight, TiO2 is a large band gap n-type semiconductor and absorbs only in the ultraviolet (UV) part of the electromagnetic spectrum. To increase absorption in the peak solar spectrum, organic dyes can be adsorbed onto the surface of TiO2, an approach that has been utilized in water splitting and also dye-sensitized solar cells.7,8 3-Fluoroaniline (3-FA) (Figure 1) is a simple precursor used for the synthesis of fine chemicals. It could be used as a dye for a photoanode as it is both colored and has the necessary functionality to bond to TiO2. It of interest in itself as a dyesensitizer; in addition, study of this molecule will broaden the knowledge of the adsorption of organic molecules onto TiO2. TiO2 is a photocatalyst and an effective material for photocatalytically degrading organic matter.10 3-FA contains a fluorine atom and also an amino group. Chlorofluorocarbons (CFCs) and hydrofluorocarbons can be photocatalytically degraded by TiO2 with UV light; fluoride ions are one of the degradation products.11−13 Amines, too, can be photocatalytically degraded with TiO2.14,15 The degradation mechanisms of © 2014 American Chemical Society

Figure 1. 3-Fluoroaniline molecule (geometry optimized by Gaussian 039). Gray spheres are carbon atoms; the dark blue sphere is the nitrogen atom; the light blue sphere is the fluorine atom; and white spheres are hydrogen atoms. The carbon atoms are numbered 1−6 for reference.

organic molecules are well-documented, including aniline16,17 and other aromatic amines,18 but these mechanisms, primarily direct photooxidation and attack by hydroxyl radicals, are related to aqueous systems.19 Any photocatalytic mechanism in a vacuum, although based on similar principles, will be subject Received: October 18, 2013 Revised: January 7, 2014 Published: January 8, 2014 2028

dx.doi.org/10.1021/jp4103405 | J. Phys. Chem. C 2014, 118, 2028−2036

The Journal of Physical Chemistry C

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Figure 2. Core XPS spectra for clean anatase TiO2(101) and 1 ML 3-FA adsorbed on anatase TiO2(101). The spectra are fitted with a linear background (dashed lines) and Voigt curves (70% Gaussian: 30% Lorentzian, dotted lines) (see text). All spectra were recorded at a photon energy of 1000 eV.

photoemission spectrum of the dosed TiO2 crystal, which we assume to be equivalent to 1 monolayer (ML) coverage. Photoemission spectra were recorded at normal emission. Core level spectra were recorded at 1000 eV photon energy (hν All binding energies (BE) (quoted to ±0.1 eV) are aligned with respect to the aromatic C peaks in the C 1s spectrum of the adsorbed 3-FA at 285 eV. A Shirley background was subtracted from the data unless stated. Voigt curves (70% Gaussian: 30% Lorentzian) were used to fit the core level spectra (with CASA XPS), consistent with previous work on these types of system.25 NEXAFS spectra were recorded at incident photon angles of 30° ≤ θ ≤ 90° relative to the surface with the electric vector of the light at 17° relative to the [010] azimuth. The NEXAFS spectra were recorded over the C K-edge using a multichannel partial yield detector, configured to detect electrons with a kinetic energy ≥250 eV. Photoemission and NEXAFS spectra were recorded with the sample at room temperature. For the photocatalytic degradation work, a UV lamp (Hg arc lamp, Emax = 4.8 eV, Lot-Oriel) was shone on the sample through a fused silica window. Valence band spectra (hν = 40 eV) were recorded while the sample was illuminated.

to the removal of fragmented molecules and radicals as they are formed, which will not be readily available for further reaction. In this paper, core and valence band photoemission results for 3-FA adsorbed on single-crystal anatase TiO2(101) are presented. We determine the orientation of the 3-FA molecule on the anatase TiO2(101) surfaces using C K-edge near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. We also examine the photodegradation of 3-FA under UV light from a mercury arc lamp. This work also adds to the adsorption studies of aniline on titania surfaces by Li et al.,20,21 Kreikemeyer-Lorenzo et al.,22 and Bradley et al.23 as well as determines the effects that fluorine, meta to the amino group, will have on the bonding environment and reactivity of 3-FA on the anatase TiO2(101) surface.

2. METHODS The work was carried out on D1011 (30 ≤ hν ≤ 1600 eV), a bending magnet soft X-ray beamline at MAX-lab in Sweden. The D1011 endstation is equipped with a SCIENTA SES200 200 mm mean-radius hemispherical electron energy analyzer, a microchannel plate (MCP) NEXAFS detector, low-energy electron diffraction (LEED) camera, and residual gas analyzer (RGA). The analysis chamber is separated from the sample preparation chamber by means of a gate valve. The anatase TiO2(101) crystal (5 mm × 5 mm, ex: Pikem Ltd.) was held in place by two strips of tantalum wire. A thermocouple was attached to the sample plate as close to the crystal as possible, allowing the sample temperature to be monitored during the experiment. The crystal was cleaned by repeated 1 keV Ar+ ion bombardment and annealing to 700 °C in vacuum until a sharp (1 × 1) LEED pattern was obtained and the X-ray photoelectron spectrum (XPS) was free from peaks due to contaminants such as C and K.24 The base pressure in the analysis chamber was around 1.5 × 10−10 mbar throughout the measurements. 3-FA (>99%, Sigma Aldrich, a clear yellow to brown liquid) was thoroughly degassed prior to use by gently heating in a glass dosing tube and venting to waste. To evaporate the 3-FA into the vacuum chamber, the dosing tube and the line to the prep chamber were heated to approximately 50 °C. Dosing was carried out via a leak valve at 1 × 10−7 mbar for 5 min (equivalent to ∼23 langmuir, where 1 langmuir = 1.3 × 10−6 mbar s). The RGA confirmed there was no degradation of the 3-FA. Dosing was carried out until there was no change in the

3. RESULTS AND DISCUSSION To study the adsorption of 3-FA on anatase TiO2(101), photoemission spectra, shown in Figure 2, were measured from the clean surface and again following adsorption of 3-FA. Figure 2a shows the C 1s spectrum for 1 ML 3-FA adsorbed on anatase TiO2(101). There are three peaks in the ratio of approximately 4:1:1 (observed ratio is 4.3:1.2:1.0), as expected for this molecule, which has carbon in three different chemical environments as summarized, along with the relative intensities, in Table 1. The C 1s spectrum therefore suggests the ring adsorbs intact on the surface. Figure 2b shows the N 1s spectrum following adsorption of 1 ML 3-FA on anatase TiO2(101). Peak assignments are shown in Table 1. The single peak observed at 400.2 eV BE is assigned to the nitrogen of the 3-FA bonded to the Ti4+ atom.21,22 Madix et al. investigated the adsorption of amines on the surface of rutile (110) and found binding energies of 400.6 eV for ammonia, 401.1 eV for dimethylamine, and 400.6 eV for ethylamine. In each case a single N 1s peak is observed, suggesting only one species of N is present following adsorption.30 Amino acids and other biomolecules adsorbed on TiO2 show some evidence of zwitterion formation because 2029



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has been measured as 689.6 eV.27 Table 2 shows the normalized peak areas of the C 1s, N 1s, and F 1s XPS

Table 1. Peak Assignment of 3-FA Adsorbed on Anatase TiO2(101)a assignment C 1s C 1s C 1s N 1s F 1s F 1s

carbon atoms 2 and 4−6 in Figure 121,26 carbon atom 1 in Figure 126 carbon atom 3 in Figure 127 nitrogen atom in Figure 121,22 elemental fluorine eV28,29 fluorine atom in Figure 127

binding energy (eV; ± 0.1 eV)

relative abundance (%)

285.0

67% of C 1s

286.2 287.3 400.2

18% of C 1s 15% of C 1s 100% of N 1s

684.2 687.5

23% of F 1s 77% of F 1s

Table 2. Stoichiometry of 3-FA on Anatase TiO2(101) Surfacea

a

Binding energies, relative abundances as a percentage of total peak area, and assignment of peaks fitted to the core spectra of 1 ML 3-FA adsorbed on anatase TiO2(101).

atom

binding energy (eV)

area

normalized area

stoichiometry

C3−6 C1 C2 N1 F1 F (element)

285.0 286.2 287.3 400.2 687.5 684.2

2463 660 571 1166 4024 1187

48 869 13 095 11 329 12 408 12 458 3 675

3.9 1.1 0.9 1.0 1.0 0.4

a

The stoichiometry of the 3-FA on the anatase TiO2(101) surface was calculated by dividing the area of the peak by the number of scans and then normalizing for the elemental cross section, as determined by Yeh and Lindau34 and the transmission functions for the Scienta analyser with 1.6 mm slits.35

two peaks are often observed in the N 1s spectrum.30−32 However, the protonated amine is generally found to be unstable with respect to heating32 and prolonged exposure to the beam.25 The nature of the binding of the nitrogen to the titanium has been the subject of much debate. Li et al. in their study of aniline on anatase TiO2(101)21 observed a single peak in the N 1s spectrum. They concluded that the N is bound to Ti4+ and the nitrogen is completely deprotonated, forming a phenyl imide upon adsorption; this is based on a comparison with the adsorption of azobenzene on the same surface. The same conclusions were also drawn by Kreikemeyer-Lorenzo et al. when adsorbing aniline onto rutile TiO2(110).22 KreikemeyerLorenzo et al. have shown via photoelectron diffraction that in aniline, the nitrogen bonds to a single titanium atom on rutile TiO2(110) rather than over two titanium atoms. More recently, a DFT study carried out by Bradley et al. has demonstrated that it is energetically unfavorable for the phenyl imide to form. The N 1s XAS spectrum of 3-FA on anatase TiO2(101) (Figure S.1 of the Supporting Information) shows there are no π* transitions, which suggests that nitrogen of the 3-FA does not exhibit any π-bonding character, although only the normal incidence spectrum was taken. On adsorption of 3-FA, the nitrogen will be positively charged, and one would expect a proton to transfer to the surface O atoms, which act as Brønsted bases10 to leave a neutral 3-FA molecule. However, the DFT calcuations by Bradley et al. have shown this not to be as energetically favorable as the adsorption of an intact aniline molecule. The amine group of 3-FA has a pKa that is lower than that aniline,33 which means that hydrogen should be removed from the 3-FA more readily than from aniline. Also, the DFT calculation by Bradley et al. is for adsorption on the rutile TiO2(110) surface rather than anatase TiO2(101); therefore, we cannot be certain as to the degree of dehydrogenation of the nitrogen of 3-FA on anatase TiO2(101), but our data does not clearly support the formation of the phenyl imide. Figure 2c shows the F 1s spectrum following adsorption of 1 ML 3-FA on anatase TiO2(101). A linear background was fitted to the F 1s spectrum as the background level was falling over this region as seen in the wide scan spectrum shown in Figure S2 of the Supporting Information. Peak assignments are shown in Table 1. There are two peaks in the F 1s spectrum, at 687.5 eV BE and 684.2 eV BE. The peak at 687.5 eV BE is assigned to the C−F on the aromatic ring, which does not take part in bonding to the surface. This is fairly consistent with the literature; the binding energy of the fluorobenzene F 1s species

peaks, adjusted for the photoionization cross sections of Yeh and Lindau34 and using calculated transmission functions for the Scienta analyzer.35 Ignoring the area of the lower binding energy F 1s peak gives an elemental ratio of 5.9:1.0:1.0 (C:N:F), which is in excellent agreement with what would be expected from the stoichiometry of the molecule (6:1:1). Thus, we assume that the molecule is predominantly bonded intact to the surface through the N group. The origin of the second fluorine peak at a binding energy of 684.2 eV is not obvious. One would expect only a single fluorine species in the F 1s spectrum for 1 ML 3-FA on anatase TiO2(101), particularly as there is only one nitrogen chemical environment. It is suggested that the peak at a binding energy of 684.2 eV arises from fluorine atoms bonded directly to the TiO2 surface. The mechanism for this is not clear. Initially, it was thought that it may be caused by electron beam damage to the molecule during LEED measurements. However, a dosed surface was measured without exposure to the LEED, and the resulting spectrum showed no obvious differences. We suggest that degradation occurs through an unstable fluorine intermediate when 3-FA interacts with the surface. Following adsorption of 1 ML 3-FA on anatase TiO2(101), a LEED pattern could not be observed. This is in contrast to azobenzene adsorption on anatase TiO2(101) where a p(2 × 1) LEED pattern is observed following fission of the molecule. The same LEED pattern was also seen following aniline adsorption.21 NEXAFS data (discussed below) suggest there is long-range ordering of 3-FA following adsorption on the anatase TiO2(101) surface, so it is possible that localized beam damage by the electron gun is occurring, preventing observation of the LEED pattern. The assignment of the peak at 684.2 eV BE to Ti−F species is consistent with literature values for Ti−F species. The fluorination of anatase TiO2 via fluoroaniline decomposition is of interest because it has been shown that fluoride ions tend to stabilize the highly photocatalytically active (001) surface of anatase TiO2.36 The use of fluorine to promote the growth of crystals of anatase TiO2 with predominantly (001) surfaces resulted in a F 1s photoelectron peak at a binding energy of 684.5 eV. In K2TiF6, the binding energy of the F 1s species has been reported as 684.937 and 685.0 eV,38 and Na2TiF6 has a binding energy of 685.3 eV.28 Yamaki et al. doped titanium with fluorine, which yields two species that are assigned to F−Ti−O with a binding 2030



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energy of 683.8 eV and Ti−F at 684.5−685.0 eV.29 Thus, the data presented here strongly suggest dissociation of some portion of the fluoroaniline molecules via C−F scission followed by formation of Ti−F at the surface. Figure 2d shows the O 1s spectra for clean anatase TiO2(101) and following adsorption of 1 ML 3-FA. Peak assignments are shown in Table 3. There are two peaks: one at Table 3. Peak Assignment of Clean Anatase TiO2(101) and Anatase TiO2(101) after Adsorption of 3-FAa

assignment

BE (eV; ± 0.1 eV); clean anatase

Ti 2p

Ti3+:2p1/244

458.2

Ti 2p

Ti4+:2p3/243

459.4

O 1s

substrate oxygen39 absorbed surface hydroxyl40

530.6

O 1s

531.7

relative abundance (%) 4% of Ti 2p 96% of Ti 2p 95% of O 1s 5% of O 1s

BE (eV; ± 0.1 eV); 3FA on anatase 457.9 459.1 530.3 531.4

relative abundance (%) 4% of Ti 2p 96% of Ti 2p 95% of O 1s 5% of O 1s

Figure 3. Valence band spectra recorded at photon energy 40 eV for (a) clean anatase TiO2(101) and (b) 3-FA adsorbed on anatase TiO2(101). The difference spectrum (c) was taken after the clean and the dosed spectra were normalized to the valence band feature at 4.4 eV.

a

Binding energies, relative abundances as a percentage of total peak area, and assignment of peaks fitted to the core spectra of clean anatase TiO2(101) before and after dosing with 1 ML 3-FA.

presence of surface defects in the form of O-vacancies.31,48 This defect peak has been ascribed to occupation of Ti 3d states after Ti3+ (d1) is created at the surface because of oxygen vacancies.45,47,49 The presence of this defect state is in agreement with the Ti 2p spectrum (Figure 2e) where a signal due to Ti3+ was also observed. This defect peak has also been attributed to Ti interstitials at the near-surface.46 Figure 3b shows the spectrum following adsorption of 1 ML 3-FA on the anatase TiO2(101) surface recorded at 110 eV and the difference spectrum (Figure 3c) obtained by subtracting the clean anatase TiO2(101) spectrum from the 3-FA-dosed anatase TiO2(101) spectrum. The spectra recorded from the clean and dosed surface were normalized to the valence band feature at 4.4 eV before subtraction. Adsorption of 3-FA leads to a number of new peaks appearing in the valence band spectrum. The defect feature at 1.0 eV binding energy observed on the clean anatase surface remains relatively unchanged following adsorption of 3-FA. Figure 4 compares the difference spectrum from Figure 3c with the UPS spectra of gaseous 3-FA, as recorded by Sky et al.50 (calibrated using a butadiene/argon/nitrogen gas mix). Figure 4b is the He I UPS spectrum and Figure 4c is the He II UPS spectrum. The gaseous 3-FA spectra have been aligned with the difference spectrum. The shift in binding energy (4.5 eV) and lower resolution in the spectrum from the adsorbed molecule are in account of comparing the spectrum recorded from a solid to those of a gas. The shift is indicative of the relaxation−polarization shift upon adsorption. This shift occurs because the final state in photoemission from the solid surface is screened by the presence of the surface.51 We assign the atomic character of the valence band states through reference to the work carried out by Sky et al., involving the ab initio calculations and experimental data for the ultraviolet photoelectron spectroscopy of 3-FA in the gaseous phase50 as shown in Table 4. The two highest occupied molecular orbitals (HOMOs) calculated by Gaussian 039 are also shown. The calculation was carried out using the B3LYP functional and 631G basis set. The two peaks in the gas phase spectra at binding

530.6 eV BE (95%), which is at a binding energy associated with the oxygen atoms in the TiO2 crystal,39 and another at 531.7 eV BE (5%), which has been assigned to adsorbed surface hydroxyls.40 The separation of approximately 1 eV agrees with that observed for a water-dosed anatase TiO2(101) surface.40 The hydroxyl signal is likely to originate from adsorption of water on the TiO2 surface from the residual vacuum. STM studies of rutile TiO2(110) surfaces suggest that surfaces are quickly hydroxylated, even at pressures as low as 10−11 Torr.41 Following adsorption of 3-FA, the O 1s signals for the anatase TiO2(101) crystal shifts 0.3 eV to a lower binding energy of 530.3 and 531.4 eV. The same shift of 0.3 eV is also observed in the Ti 2p spectra discussed below. Similar shifts are observed for dopamine adsorption on anatase TiO2(101).42 The relative intensities of the two peaks in the O 1s spectrum are unchanged after adsorption of 3-FA. Figure 2e shows the Ti 2p spectra for clean anatase TiO2(101) and following adsorption of 1 ML 3-FA. Peak assignments are shown in Table 3. The clean Ti 2p spectrum consists of the spin orbit split Ti 2p3/2 and Ti 2p1/2.43 Peak fitting is carried out only on the Ti 2p3/2, which has a binding energy of 458.2 eV. There is also evidence of residual Ti3+ at 459.4 eV BE44 (4% of the peak area), which is attributed to residual surface oxygen vacancies45 or subsurface oxygen vacancies.46 Following adsorption of 3-FA, the Ti 2p peaks shift 0.3 eV to lower binding energy. These shifts correlate with the shifts observed in the O 1s spectra following adsorption of 3-FA. The size of the Ti3+ peak remained unchanged after dosing 3-FA. Figure 3a shows valence band spectra for clean anatase TiO2(101) recorded at 40 eV photon energy. The main valence band structure in the clean anatase spectrum has peaks at 4.4 eV, 6.1 and 7.8 eV BE, which is consistent with previous work.47 The valence band structure arises from O 2p states with some Ti 3d and 4sp character. The clean anatase TiO2(101) spectrum has a very weak peak at ∼1 eV BE, indicating the 2031



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a single peak in the adsorbed molecule spectrum is a result of a slight shift to higher binding energy of the 3.8 eV peak along with broadening of both features. The shift in binding energy is a result of the bonding through the amine group to the surface. Similar shifts have been observed in water adsorption on oxide surfaces.52 Figure 5a shows the carbon K-edge NEXAFS spectra of 1 ML 3-FA adsorbed on anatase TiO2(101) for incident radiation angles of 30−90° to the surface. The spectra are first normalized in order to remove photon flux and substrateinduced features by dividing the 3-FA-dosed anatase TiO2(101) carbon K-edge NEXAFS spectrum by the corresponding clean anatase TiO2(101) spectrum. This is necessary because the energy cutoff of 250 eV used in the partial yield detector will still allow the detection of Ti 3s and Ti 3p photoemission peaks. The resulting spectra are subsequently normalized by setting the height of the step edge (the increase in intensity when passing the ionization potential) to unity. Sharp shape resonances due to C 1s → π* excitations are seen at photon energies below 288 eV. These peaks are assigned as follows: the peak at 284.7 eV photon energy (peak a) in Figure 5a arises from excitations from the C 1s level into the π* orbitals of carbon atoms 2, 4−6 (C−C/C−H).26 That at 285.9 eV (peak b) originates from C 1s → π* excitations for carbon atom 1 (C−N).55 The peak at 287.1 eV (peak c) originates from C 1s → π* excitations for carbon atom 3 (C−F).56 The peaks have a height ratio of approximately 4:1:1, as expected. All three peaks exhibit a degree of asymmetry which is due to excitation from the chemically shifted C 1s states described above into the π* lowest unoccupied molecular orbital (LUMO) and LUMO+1 states. Figure 5b shows the detail in the NEXAFS spectrum at normal incidence as well as the first two LUMOs, as calculated by Gaussian 03.9 The inset in Figure 5a is normalized to peak (a) intensity recorded with the incident beam 90° to the surface. Stöhr equations57 were fitted to the data points. The fit gives an angle for the π* vector-like orbital parallel to the surface (i.e., the molecule is oriented with the plane of the phenyl ring perpendicular to the surface) and twisted roughly 10 ± 5° from the ⟨010⟩ azimuth (shown in Figure 5c). For aniline, the DFT calculations of Bradley et al.23 and the experimental work of Kreikemeyer-Lorenzo et al.22 suggests that the plane of the ring is tilted away from the surface normal. The data for 3-FA strongly supports the molecule in a perpendicular orientation with alignment just off the ⟨010⟩ azimuth. The differences in orientation between aniline and 3FA are likely due to the electrostatic interactions induced by the fluorine of the 3-FA In order to investigate the suitability of 3-FA as a sensitizer and also to determine the photostability of the molecule on the anatase TiO2(101) surface, measurements were carried out with the sample under illumination from an intense UV−visible lamp. The lamp has a peak energy output of around 4.8 eV (ca. 260 nm) and was shone through a fused silica viewport, ensuring around 90% transmission of wavelengths from 260 to 2000 nm (4.8 to 0.6 eV). Figure 6 shows the valence band spectra (40 eV) of 3-FA on the anatase TiO2(101) surface under illumination with this source. Figure 6a is the spectrum before UV irradiation. Figure 6b is recorded immediately after switching on the source; after significant change in the spectrum was noticed, i.e., features associated with the 3-FA disappearing, the lamp was switched off and another spectrum taken (Figure 6c). No more changes to the valence band were

Figure 4. Comparison of the difference spectrum of clean anatase TiO2(101) and 3-FA adsorbed on anatase TiO2(101) (a) and the UPS spectra of gaseous 3-FA. Panel (b) is the He I spectrum and (c) is the He II spectrum as recorded by Sky et al. Gaseous 3-FA spectra were reproduced from ref 50.

Table 4. Peak Assignment of the Difference Spectra of 3-FA Adsorbed onto Anatase TiO2(101) and Clean Anatase TiO2(101)a

a

Peak assignments are based on work carried out by Sky et al., involving ab initio calculations and experimental data for the ultraviolet photoelectron spectroscopy analysis of 3-FA in the gaseous phase,50 and includes the eight lowest calculated molecular orbitals. The molecular orbitals (MOs) shown here were calculated using the B3LYP functional and 6-31G basis set.

energies of 3.8 and 4.8 eV both have some density of states on the amine group. We believe the fact that these peaks appear as 2032



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Figure 5. (a) Carbon K-edge NEXAFS spectra of 3-FA adsorbed on anatase TiO2(101) surface. The inset graph shows the resulting height of the πa* peaks plotted as a function of angle of photon incidence to the surface, with the Stöhr equations57 fitted to the data points. (b) Zoomed in view of carbon K-edge NEXAFS spectra of 3-FA adsorbed on anatase TiO2(101) surface at normal incidence. Peak a corresponds to carbons 2 and 4−6 of figure 1 (C−H), peak B to carbon 1 (C−N), and peak C to carbon 3 (C−F). Multiple components can clearly be seen in all three peaks which correspond to transitions to the lowest unoccupied molecular orbitals, displayed in the inset (calculated by Gaussian 039). (c) A 3-FA molecule adsorbed on an anatase TiO2(101) cluster. For the anatase TiO2(101) crystal, titanium is represented by light blue and oxygen is represented by red atoms. In the 3-FA molecule, carbon atoms are gray, nitrogen dark blue, fluorine light blue, and hydrogen white.

observed when the lamp was turned on again, signifying no further change on the surface. Although, as can be seen by comparison between spectra (c) and (d) in Figure 6, the 3-FAdosed surface does not return to that of the clean surface upon UV irradiation, it appears most of the structure associated with the presence of the molecule is removed. To investigate the observed effects further, the core level spectra were recorded immediately following illumination with the UV−visible lamp and are shown in Figure 7a−c. No nitrogen is detected in the region of the N 1s spectrum, indicating complete fission of the N−Ti bond. There is still carbon on the surface, albeit greatly reduced. The C 1s peaks have not been fitted because of uncertainty of the origin of the peaks (they have binding energies similar to the 3-FA, but it is impossible to determine accurately the nature of the degradation products) and the poor signal-to-noise ratio. The presence of C species may be the reason for the higher background in the valence band spectrum and also slight differences between the “photocatalytically degraded” 3-FA spectra (b) and (c) in Figure 7 as compared to the clean spectrum. The F 1s spectrum shows that the peak at 687.5 eV BE, attributed to the intact molecule, is no longer present. This is in agreement with the N 1s spectrum because if the N−Ti bond is broken, the whole molecule desorbs from the surface. The fluorine species at 684.2 eV BE remains, suggesting it is indeed bound directly to the surface in the form of TiF. With regard to the degradation of the molecule, there are two processes that have been observed in this work: (1) the fission of the C−F bond, leaving fluorine on the surface; and (2) UV-

Figure 6. Photodegradation of 3-FA adsorbed on anatase TiO2(101) surface monitored through the valence band spectra (40 eV). (a) 3fluoroaniline-dosed anatase TiO2(101) before UV irradiation; (b) under UV radiation; and (c) subsequent scan with the UV lamp switched off. Spectrum (d) is the clean anatase TiO2(101) for comparison.

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Figure 7. Core levels of 3-FA adsorbed on anatase TiO2(101) after photodegradation induced by the UV lamp (Hg, 4.8 eV). All spectra were recorded at 1000 eV. Panel (a) is the C 1s spectrum, (b) the N 1s spectrum, and (c) the F 1s spectrum.

induced fission of the Ti−N bond, desorbing 3-FA from the surface. Considering the former, the loss of fluoride in fluoridecontaining aromatics has been reported as a result of beaminduced ionization.58 It has been well-documented that a fluorine atom can be dissociated from a phenyl ring by hydroxide radical attack on the aromatic ring;59−61 however, this work was carried out in aqueous environments. On the other hand, even in UHV, there are hydroxyls present on the anatase TiO2 surface41 which could be radicalized by the electron beam and then react with the carbon bonded to the fluorine, displacing the fluorine with the hydroxyl. However, this would likely lead to an increase in the defect peak intensity because of the formation of an oxygen vacancy, which is not observed in the valence band spectra.47 Nonphotocatalytic defluorination of fluorobenzene has been demonstrated on a Rh/Al2O3 catalyst system, and the mechanism for the rapid degradation began with loss of fluoride.62 For the Ti−F bond to be formed, the first step is the formation of a positive divalent fluorine species, which will be relatively unstable, possibly forming only through stabilization induced by TiO2 surface electrons. The strength of the Ti−F bond and the electronegative fluorine induces breaking of the C−F bond, and the aniline fragment is removed to the vacuum. As for the UV−vis induced Ti−N fission, again it is the question whether the degradation mechanism is through direct hole oxidation or via neighboring hydroxyl radicals produced by the UV light. The photocatalytic degradation of aniline was carried out on TiO2 in aqueous solutions with the main mechanism being hydroxide radical attack on the benzene ring. The study was also carried out with t-BuOH, which scavenges the hydroxyl radicals, showing that photodegradation can also take place via positive hole oxidation.63 As in the C−F degradation, if the photodegradation mechanism occurs via hydroxyl radicals, then a change in the defect state would likely be seen. Because we cannot definitively identify the adsorption mode, it is difficult to unambiguously predict the photodegradation mechanism. For example, if the 3-FA molecule bonds intact as aniline as suggested by Bradley et al.,23 resulting in a positively charged N in the molecule upon adsorption, then the 3-FA will be a good leaving group upon photoreduction, i.e., electron transfer from the substrate. On the other hand, if phenyl imide is formed, then the photodesorption would occur via photooxidation of the molecule, i.e., electron transfer from the molecule to the substrate.

4. CONCLUSIONS 3-FA was found to adsorb on the TiO2(101) anatase surface through the nitrogen to the 5-fold coordinated Ti atom. It also bonds independently through the fluorine atom, but degrades to leave fluorine on the surface. Angle-resolved NEXAFS spectra show the plane of the ring of the molecule to be tilted at 90 ± 5° away from the surface (i.e., the molecule is upright on the surface) and twisted roughly 10 ± 5° off the ⟨010⟩ direction (see Figure 5c). When irradiated with UV light (4.8 eV), 3-FA is found to be completely desorbed from the surface, following fission of the N−Ti bond.

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ASSOCIATED CONTENT

S Supporting Information *

N K-edge NEXAFS spectra (Figure S.1) showing the lack of N 1s to π* associated resonances and a survey spectrum of the 3FA dosed TiO2 surface (Figure S.2) showing a decrease in intensity of the background over the F 1s region. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank MAX-lab for the award of experimental beam time funded by the Swedish Research Council. M.J.J. thanks EPSRC (UK) for the award of a studentship through the NowNano Doctoral Training Centre (Grant EP/G03737X/1). A.T. thanks the Photon Science Institute and The University of Manchester for funding. The authors are grateful to Alexei Preobrajenski for training and technical advice on use of the beamline.

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ABBREVIATIONS 3-FA: 3-Fluoroaniline NEXAFS: Near-edge X-ray adsorption fine structure LEED: Low-energy electron diffraction XPS: X-ray photoelectron spectroscopy UPS: Ultraviolet photoelectron spectroscopy dx.doi.org/10.1021/jp4103405 | J. Phys. Chem. C 2014, 118, 2028−2036


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Adsorption and Photocatalytic Degradation of 3-Fluoroaniline on

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