pubs.acs.org/Langmuir © 2010 American Chemical Society
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Dopamine Adsorption on Anatase TiO2(101): A Photoemission and NEXAFS Spectroscopy Study K. Syres,† A. Thomas,*,† F. Bondino,‡ M. Malvestuto,§ and M. Gr€atzel †
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School of Physics and Astronomy, The Photon Science Institute, Alan Turing Building, The University of Manchester, Oxford Road, Manchester M13 9PL, U.K., ‡IOM CNR, Laboratorio TASC, Strada Statale 14 Km. 163,5, I-34149 Basovizza, Trieste, Italy, §Sincrotrone Trieste S.CpA di Interesse Nazionale, Strada Statale 14 Km. 163,5 in Area Science Park, 34149 Basovizza, Trieste, Italy, and Laboratory of Photonics and Interfaces, Institute of Chemical Science and Engineering, Faculty of Basic Science, Ecole Polytechnique F ed erale de Lausanne, CH-1015 Lausanne, Switzerland Received April 22, 2010. Revised Manuscript Received August 4, 2010 The adsorption of dopamine onto an anatase TiO2(101) single crystal has been studied using photoemission and NEXAFS techniques. Photoemission results suggest that the dopamine molecule adsorbs on the surface in a bidentate geometry, resulting in the removal of band gap states in the TiO2 valence band. Using the searchlight effect, carbon K-edge NEXAFS spectra indicate that the phenyl rings in the dopamine molecules are orientated normal to the surface. A combination of experimental and computational results indicates the appearance of new unoccupied states arising following adsorption. The possible role of these states in the charge-transfer mechanism of the dopamine-TiO2 system is discussed.
1. Introduction TiO2 is of technological interest in applications including photocatalysis, biomaterials, and novel photovoltaic solar cells.1-4 Many of these applications use TiO2 in a nanoparticulate form. TiO2 is used as an n-type semiconductor in dye-sensitized solar cells (DSSCs). Because TiO2 absorbs light in the UV region, in DSSCs the surface is coated with an organic molecule that absorbs in the visible region of the spectrum.4,5 TiO2 is also highly biocompatible,3,6 which makes it ideal for use in medical applications. Titanium implants have been found to be very successful because of the TiO2 layer that forms at their surfaces.3,6 TiO2functionalized nanoparticles have been studied for a number of biological and environmental applications including bioelectronics, antifouling materials, and killing bacteria.1,7,8 Dopamine, shown in Figure 1, is often used in these systems to modify the surfaces of TiO2 nanoparticles where it is used as an anchor molecule to which other molecules, such as polymer chains may (1) Chen, W. J.; Tsai, P. J.; Chen, Y. C. Functional Fe3O4/TiO2 core/shell magnetic nanoparticles as photokilling agents for pathogenic bacteria. Small 2008, 4, 485–491. (2) Chen, Z. Y.; Hu, Y.; Liu, T. C.; Huang, C. L.; Jeng, T. S. Mesoporous TiO2 thin films embedded with Au nanoparticles for the enhancement of the photocatalytic properties. Thin Solid Films 2009, 517, 4998–5000. (3) Tengvall, P.; Lundstrom, I. Physico-chemical considerations of titanium as a biomaterial. Clin. Mater. 1992, 9, 115–134. (4) O’Regan, B.; Gr€atzel, M. A low-cost, high-efficiency solar-cell based on dyesensitized colloidal TiO2 films. Nature 1991, 353, 737–740. (5) Nazeeruddin, M. K.; Kay, A.; Rodicio, I.; Humphrybaker, R.; Muller, E.; Liska, P.; Vlachopoulos, N.; Gr€atzel, M. Conversion of light to electricity by cisX2bis(2,20 -bipyridyl-4,40 -dicarboxylate)ruthenium(II) charge-transfer sensitizers (X = Cl-, Br-, I-, Cn-, and SCN-) on nanocrystalline TiO2 electrodes. J. Am. Chem. Soc. 1993, 115, 6382–6390. (6) Kasemo, B.; Lausmaa, J. Surface science aspects on inorganic biomaterials. Crit. Rev. Biocompat. 1986, 2, 335–380. (7) Lee, H.; Scherer, N. F.; Messersmith, P. B. Single-molecule mechanics of mussel adhesion. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 12999–13003. (8) Liu, J. Q.; de la Garza, L.; Zhang, L. G.; Dimitrijevic, N. M.; Zuo, X. B.; Tiede, D. M.; Rajh, T. Photocatalytic probing of DNA sequence by using TiO2/ dopamine-DNA triads. Chem. Phys. 2007, 339, 154–163. (9) Vega-Arroyo, M.; LeBreton, P. R.; Zapol, P.; Curtiss, L. A.; Rajh, T. Quantum chemical study of TiO2/dopamine-DNA triads. Chem. Phys. 2007, 339, 164–172.
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be grafted. The dopamine molecules can also facilitate charge transfer between the TiO2 nanoparticles and the biological system.9,10 Chen et al.1 studied a system capable of inhibiting the cell growth of particular pathogenic bacteria (bacteria that cause diseases) when illuminated with UV light. This system is composed of nanoparticles with an iron oxide core and a titanium dioxide shell. Dopamine molecules were self-assembled onto the TiO2 shell and used to anchor succinic anhydride onto the surface. This in turn was used to attach immunoglobulin G (IgG) to the nanoparticles. TiO2 nanoparticles with DNA anchored onto the nanoparticle surfaces via dopamine have been studied for applications of DNA recognition.8,10 These systems could potentially provide the basis of sensors for DNA hybridization (a DNA recognition technique) and for the detection of proteins binding to DNA. In these potential biomedical applications, the orientation and stability of the adsorbates are clearly important. Catechol adsorption on TiO2 is of interest as a light-harvesting molecule for solar cells.11 Catechol does not absorb light below 4.2 eV (300 nm), which is much larger than the 3.2 eV (370 nm) band gap of TiO2. However, catechol-sensitized TiO2 nanoparticles have an absorption onset of around 3 eV (420 nm).12 This shift has been found to differ depending on the specific catechol molecule used.13 The molecule-to-surface charge transfer in this system is thought to differ from that in DSSCs. In the DSSC systems mentioned above, the dye absorbs the incident photon, resulting in electron hole separation in the dye.4 The electron then undergoes rapid transfer to the TiO2 conduction band. In the case (10) Rajh, T.; Saponjic, Z.; Liu, J. Q.; Dimitrijevic, N. M.; Scherer, N. F.; VegaArroyo, M.; Zapol, P.; Curtiss, L. A.; Thurnauer, M. C. Charge transfer across the nanocrystalline-DNA interface: probing DNA recognition. Nano Lett. 2004, 4, 1017–1023. (11) Duncan, W. R.; Prezhdo, O. V. Theoretical studies of photoinduced electron transfer in dye-sensitized TiO2. Annu. Rev. Phys. Chem. 2007, 58, 143–184. (12) Persson, P.; Bergstrom, R.; Lunell, S. Quantum chemical study of photoinjection processes in dye-sensitized TiO2 nanoparticles. J. Phys. Chem. B 2000, 104, 10348–10351. (13) Creutz, C.; Chou, M. H. Binding of catechols to mononuclear titanium(IV) and to 1-and 5-nm TiO2 nanoparticles. Inorg. Chem. 2008, 47, 3509–3514.
Published on Web 08/24/2010
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Figure 1. (Left) Dopamine molecule with carbon atoms labeled from 1 to 8 for reference throughout the article. (Right) Geometry-optimized cluster model of dopamine on an anatase TiO2(101) surface. Red spheres represent O atoms, gray spheres represent carbon atoms, small white spheres represent H atoms, blue spheres represent N atoms, and light-gray spheres represent Ti atoms.
of the catechol-TiO2 system, however, it has been proposed by Persson et al.12 that instead of the electron being excited in the adsorbate and then being transferred into the TiO2 conduction band there is a direct catechol-to-TiO2 charge transfer (i.e., the electron is directly photoinjected from catechol into the conduction band of TiO2 without the participation of excited states in catechol). This direct charge transfer is believed to be an excitation from the highest occupied π orbital in catechol to the Ti4þ (3d) levels at the bottom of the conduction band of TiO2.12 Vega-Arroyo et al.14 have performed theoretical calculations of dopamine on anatase TiO2 defect sites and have concluded that the dopamine molecule would adsorb in a chelating bidentate structure. Vega-Arroyo et al. in their study of dopamine also calculated optical shifts for monodentate and bidentate structures. The calculated optical red shift (1.2-1.6 eV) of the bidentate structure is similar to the experimentally observed optical shift and to catechol adsorption onto TiO2. On a stoichiometric surface, rather than in a defect state, dopamine is expected to adsorb in a bridging bidentate geometry as has been determined theoretically for catechol adsorption on anatase TiO2 by Redfern et al.15 Nanoparticle surfaces generally adopt the anatase (101) structure because this is the lowest-energy surface for TiO2.16 Li et al.17 have studied catechol adsorbed on rutile TiO2(110). Scanning tunnelling microscopy (STM) was used to elucidate the bonding geometry of catechol on the surface, and UV photoemission was used to investigate the presence of band gap states in TiO2. DFT calculations were used to help understand the experimental results. It was found that only when catechol is adsorbed in a bridging bidentate geometry does it introduce states into the band gap of the TiO2. Using angle-resolved UPS, they calculated that the catechol molecules that give rise to the band gap state are tilted 15-30° from the surface normal. (14) Vega-Arroyo, M.; LeBreton, P. R.; Rajh, T.; Zapol, P.; Curtiss, L. A. Density functional study of the TiO2-dopamine complex. Chem. Phys. Lett. 2005, 406, 306–311. (15) Redfern, P. C.; Zapol, P.; Curtiss, L. A.; Rajh, T.; Thurnauer, M. C. Computational studies of catechol and water interactions with titanium oxide nanoparticles. J. Phys. Chem. B 2003, 107, 11419–11427. (16) Gong, X. Q.; Selloni, A.; Batzill, M.; Diebold, U. Steps on anatase TiO2(101). Nat. Mater. 2006, 5, 665–670. (17) Li, S. C.; Wang, J. G.; Jacobson, P.; Gong, X. Q.; Selloni, A.; Diebold, U. Correlation between bonding geometry and band gap states at organic-inorganic interfaces: catechol on rutile TiO2(110). J. Am. Chem. Soc. 2009, 131, 980–984.
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In this article, photoemission results of dopamine on singlecrystal anatase TiO2(101) are presented. From this data, the bonding mechanism of the dopamine molecule on the anatase TiO2(101) surface is elucidated. Carbon K-edge NEXAFS spectroscopy results are presented, and the orientation of dopamine molecules on the TiO2 surface is determined. Experimental NEXAFS spectra are compared to simulated NEXAFS spectra of the isolated dopamine molecule and dopamine on an anatase TiO2(101) cluster. The calculations are also used to interpret the NEXAFS spectra, and an attempt is made to elucidate the chargetransfer process.
2. Methods 2a. Experimental Section. The work was carried out on beamline Bach at the Elettra synchrotron facility in Trieste, Italy. The beamline has a photon energy range of 35-1600 eV. The Bach end station is fitted with a 150 mm mean radius hemispherical electron energy analyzer with a 16-channel detector. The anatase TiO2(101) crystal (approximately 3 mm 3 mm) was grown by a chemical transport method.18 It was mounted by being wrapped in a Ta strip, which was then spot-welded across the sample plate, and a thermocouple was spot-welded to the tantalum strip to allow the sample temperature to be monitored. The crystal was cleaned by repeated 1 keV Arþ ion etching and annealing to 750 °C in vacuum until the XPS spectra showed no contamination and a sharp 1 1 LEED pattern was obtained.19 The base pressure in the end station was around 5 10-10 mbar. Dosing of the anatase single crystal surface was carried out by packing dopamine hydrochloride (3-hydroxytyramine hydrochloride, g98.5% Sigma Aldrich) into a tantalum envelope and heating. The powder was degassed prior to use by heating the tantalum envelope in the evaporating arm for 1 h. To evaporate dopamine into the chamber, the temperature of the tantalum envelope was held at approximately 180 °C. (The anatase TiO2 crystal was at approximately -130 °C.) A quadrupole mass spectrometer located in the analysis chamber was used to monitor the evaporation. It was found that dopamine hydrochloride decomposed on heating to form dopamine (mass 153) and HCl (mass 36). Dopamine was evaporated into the chamber for 10 min, (18) Hengerer, R.; Bolliger, B.; Erbudak, M.; Gratzel, M. Structure and stability of the anatase TiO2 (101) and (001) surfaces. Surf. Sci. 2000, 460, 162–169. (19) Thomas, A. G.; Flavell, W. R.; Kumarasinghe, A. R.; Mallick, A. K.; Tsoutsou, D.; Smith, G. C.; Stockbauer, R.; Patel, S.; Gratzel, M.; Hengerer, R. Resonant photoemission of anatase TiO2 (101) and (001) single crystals. Phys. Rev. B 2003, 67, 3.
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Table 1. Comparison of Binding Energies with the Literature and Comparison of the Expected Stoichiometry Percentage with the Actual Percentage for C 1s and O 1s Peak Fitting for Dopamine Adsorbed onto Anatase TiO2(101)a percentage
expected
experiment
C 3-7 62.5 61.5 C 1-2,8 37.5 38.5 60.4 O (TiO2) O (dop)-Ti 35.6 O (OH) 4.1 a C1-8 correspond to carbon atoms Figure 1.
binding energy (eV) literature (on similar systems)
dopamine/ TiO2
284.537 284.5 285.9 286.137 530.2, 530.124,25 530.5 531.6 531.525 25,26 533.3 533.1, 533.7 in the dopamine molecule in
resulting in a rise in pressure in the main chamber to 7 10-6 mbar during the dosing process. This resulted in a thick multilayer adsorbed onto the anatase surface. XPS spectra recorded from this thick film were found to be free of chloride species, suggesting that only dopamine was adsorbed. Heating the crystal to ∼30 °C removed the multilayer and resulted in just over a monolayer of coverage as determined from the O 1s and N 1s spectra.20 Photoemission spectra were recorded at normal emission with the incident beam positioned at 60° relative to the surface normal. Core-level spectra were recorded at a photon energy of 590 eV, and valence band spectra were recorded at 453 eV photon energy. Binding energies are referenced to the C 1s signal from the ring at 284.5 eV as recorded from the dopamine-dosed surface and quoted to (0.1 eV. NEXAFS spectra were recorded at incident photon angles of 30° e θ e 90° relative to the surface. The NEXAFS spectra were recorded by detecting C Auger electrons at a kinetic energy of 260 eV using the hemispherical analyzer. During the recording of photoemission and NEXAFS spectra, the sample temperature was held at around 25-30 °C. Peak fitting was performed using CasaXPS software. A Shirley background was subtracted from all of the core-level photoemission data, which resulted in the removal of all noise outside the fitted region. Voigt curves (70:30% Gaussian/Lorentzian) were used to fit the core-level spectra. A third-order polynomial background was subtracted from the valence band spectra. 2b. Computer Modeling. Computer modeling was carried out using Gaussian ’0321 and StoBe-deMon.22 Gaussian ’03 was used to perform geometry optimizations of the isolated dopamine molecule, and the dopamine molecule adsorbed onto anatase TiO2(101). StoBe-deMon software was used to perform excitedstate calculations of these systems for NEXAFS simulations. A geometry optimization of the dopamine molecule was done using Gaussian ‘03 utilizing DFT B3LYP theory and a 6-31G(d,p) basis set. The coordinates of the geometry-optimized molecule (20) Patthey, L.; Rensmo, H.; Persson, P.; Westermark, K.; Vayssieres, L.; Stashans, A.; Petersson, A.; Bruhwiler, P. A.; Siegbahn, H.; Lunell, S.; Martensson, N. Adsorption of bi-isonicotinic acid on rutile TiO2(110). J. Chem. Phys. 1999, 110, 5913–5918. (21) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, revision C.02; Gaussian, Inc.: Wallingford, CT, 2004. (22) Hermann, K.; Pettersson, L. StoBe-deMon software; Stockholm-Berlin version 2.2 of deMon, 2006.
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were transferred into StoBe to generate a NEXAFS spectrum. The eight carbon atoms (numbered in Figure 1) were calculated separately, energy calibrated, and then added to produce the total NEXAFS simulation. To model the dopamine molecule on the anatase TiO2(101) surface, a cluster of five titanium atoms was used, as shown in Figure 1. Hydrogen saturators were used on oxygen atoms with dangling bonds, and DFT theory with the B3LYP exchange correlation functional and a 6-31G basis set was used for the geometry optimization. The coordinates of the atoms in the TiO2 cluster were kept frozen during the calculation, except for the two titanium atoms to which the dopamine is bonded and the oxygen atom between these two titanium atoms. StoBe was then used to generate a NEXAFS spectrum from the new coordinates.
3. Results Figure 2 shows the O 1s, C 1s, and N 1s spectra for clean anatase TiO2(101) and following the adsorption of ∼1 ML of dopamine on the anatase TiO2(101) surface. In the O 1s spectrum, clean anatase shows two peaks at 530.8 eV (93.3%) and 532.2 eV (6.7%). The largest peak arises from the oxygen atoms on the TiO2(101) surface. The other peak could arise from oxygen atoms that are close to oxygen vacancy sites. It could also be due to hydroxyl groups on the surface.23 The O 1s spectrum following the adsorption of dopamine on anatase was fitted with peaks at 530.5 eV (60.4%), 531.6 eV (35.6%), and 533.3 eV (4.1%). The most intense peak corresponds to the oxygen atoms on the TiO2 surface.24,25 The peak at 531.6 eV is assigned to photoemission from the deprotonated oxygen in the dopamine molecule adsorbed onto the surface as C-O-Ti by comparison with carboxylic acids adsorbed onto rutile TiO2(110).25 The very small third peak likely arises from hydroxyl groups, either from contamination during dosing26 or from dopamine that is not directly adsorbed onto the TiO2 surface.25 From the O 1s spectrum, it is deduced that both oxygen atoms in the dopamine molecule that are chemisorbed to the surface are in the same environment, which implies that bonding to the surface is through both oxygen atoms following deprotonation. However, we note that the small peak attributed to hydroxyl may be indicative of dopamine molecules adsorbed through only one hydroxyl group, as has been seen in pyrocatechol (a related molecule) adsorbed onto rutile TiO2(110).17 The adsorption through two oxygen atoms supports theoretical calculations of catechol on anatase TiO2 by Redfern et al.,15 who found that catechol should adsorb in a bridging bidentate geometry with each oxygen atom attached to different titanium atoms. However, the presence of only one oxygen species associated with the adsorbed dopamine in our experimental data does not rule out the possibility that both oxygen atoms bond to one Ti atom (bidentate chelating). This binding mechanism has been suggested to dominate at O vacancy sites by Vega-Arroyo et al. 14 The C 1s spectrum following the adsorption of dopamine on the anatase surface can be fitted with two peaks at 284.5 eV (61.5%) (23) Wendt, S.; Matthiesen, J.; Schaub, R.; Vestergaard, E. K.; Laegsgaard, E.; Besenbacher, F.; Hammer, B. Formation and splitting of paired hydroxyl groups on reduced TiO2(110). Phys. Rev. Lett. 2006, 96, 6. (24) Huang, N. P.; Michel, R.; Voros, J.; Textor, M.; Hofer, R.; Rossi, A.; Elbert, D. L.; Hubbell, J. A.; Spencer, N. D. Poly(L-lysine)-g-poly(ethylene glycol) layers on metal oxide surfaces: surface-analytical characterization and resistance to serum and fibrinogen adsorption. Langmuir 2001, 17, 489–498. (25) Mayor, L. C.; Ben Taylor, J.; Magnano, G.; Rienzo, A.; Satterley, C. J.; O’Shea, J. N.; Schnadt, J. Photoemission, resonant photoemission, and X-ray absorption of a Ru(II) complex adsorbed on rutile TiO2 (110) prepared by in situ electrospray deposition. J. Chem. Phys. 2008, 129, 11. (26) Thomas, A. G.; Flavell, W. R.; Chatwin, C. P.; Kumarasinghe, A. R.; Rayner, S. M.; Kirkham, P. F.; Tsoutsou, D.; Johal, T. K.; Patel, S. Adsorption of phenylalanine on single crystal rutile TiO2(110) surface. Surf. Sci. 2007, 601, 3828– 3832.
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Figure 2. Oxygen 1s, carbon 1s, and nitrogen 1s photoemission spectra recorded from clean and dopamine-dosed anatase TiO2(101). Spectra were recorded at a photon energy of 590 eV. The dashed lines are 70:30 Gian/Lorentzian peaks fitted to the experimental data.
and 285.9 eV (38.5%). The peak at lower binding energy is from carbon atoms 3-6 and 7 in the dopamine molecule (Figure 1). The peak at higher binding energy is from carbon atoms 1, 2, and 8 in the dopamine molecule (Figure 1) (i.e., carbon atoms that are attached to oxygen or nitrogen atoms). The area ratio of the peaks in the experimental data is 1.6:1. Theoretically, a ratio of approximately 1.7:1 would be expected. The reason for the discrepancy between the experimental data and the expected ratios is not clear. C 1s spectra recorded before and after the NEXAFS measurements indicate no observable changes in the relative intensities of these two peaks, suggesting that the molecule is reasonably stable when adsorbed on the anatase surface under illumination with synchrotron radiation up to 1000 eV. The N 1s spectrum following the adsorption of dopamine on the anatase TiO2(101) surface, shown in Figure 2, is fitted with two peaks at 399.8 eV (51.1%) and 402.2 eV (48.9%). The presence of nitrogen suggests that the molecule remains intact at the surface because nitrogen lies at the terminal end of the side chain. The peak at 399.8 eV is assigned to the NH2 group of the dopamine molecule. The origin of the second peak is less clear. The binding energy of this peak and its separation from the peak assigned to NH2 are consistent with that of the NH3þ species that has been observed in the photoemission of amino acid powders.27 However, the loss of nitrogen from the side chain and the formation of ammonia or methyl amine, for example, cannot be ruled out in the data presented here, and these could in some way explain the difference between the theoretical and experimental carbon ratios described above. Figure 3 shows the Ti 2p and valence band spectra for the clean anatase TiO2(101) surface and following the adsorption of dopamine on the surface. The spin-orbit-split 2p3/2 and 2p1/2 components can be clearly seen in the Ti 2p spectrum at binding energies of 459.2 and 464.0 eV. It can be seen that the 2p3/2 component has a shoulder at a lower binding energy. Only the 2p3/2 component has been fitted with peaks at 457.5 eV (28.2%) and 459.2 eV (71.8%). The peak at 459.2 eV arises from the majority of surface Ti4þ ions. The peak at 457.5 eV is due to Ti3þ at the surface that (27) Zubavichus, Y.; Fuchs, O.; Weinhardt, L.; Heske, C.; Umbach, E.; Denlinger, J. D.; Grunze, M. Soft X-ray-induced decomposition of amino acids: An XPS, mass spectrometry, and NEXAFS study. Radiat. Res. 2004, 161, 346–358.
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Figure 3. (Left) Titanium 2p photoemission spectra for clean and dopamine-dosed anatase TiO2(101) recorded at 590 eV photon energy. The dashed lines are 70:30 Gian/Lorentzian peaks fitted to the Ti 2p3/2. (Right) TiO2 valence band spectra recorded at a photon energy of 453 eV. The difference spectrum obtained by subtracting the clean spectrum from the dopamine-dosed spectrum is also shown.
arises from the presence of surface oxygen vacancies28 and indicates an appreciable concentration of Ti3þ at or near the surface. The dopamine-dosed anatase Ti 2p3/2 spectrum is fitted with peaks at 457.3 eV (23.9%) and 459.0 eV (76.1%). The main difference between the clean and dosed surfaces is that the peak at 457.3 eV that arises from Ti3þ at the surface is slightly reduced in intensity following the adsorption of dopamine. (28) Thomas, A. G.; Flavell, W. R.; Mallick, A. K.; Kumarasinghe, A. R.; Tsoutsou, D.; Khan, N.; Chatwin, C.; Rayner, S.; Smith, G. C.; Stockbauer, R. L.; Warren, S.; Johal, T. K.; Patel, S.; Holland, D.; Taleb, A.; Wiame, F. Comparison of the electronic structure of anatase and rutile TiO2 single-crystal surfaces using resonant photoemission and X-ray absorption spectroscopy. Phys. Rev. B 2007, 75, 3.
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The clean valence band spectrum recorded at a photon energy of 453 eV, shown in Figure 3, is consistent with earlier work on the anatase TiO2(101) surface19 although the main valence band peaks at binding energies of 5 and 8 eV are not well-resolved. The intensity of the band gap state at 1 eV binding energy is comparable to that produced following sputtering and vacuum annealing cycles in earlier work28 and has been attributed to the presence of surface oxygen vacancies in both anatase and rutile crystals.28,29 A difference spectrum obtained by subtracting the clean spectrum from the dopamine-dosed surface is also shown in Figure 3 in order to clarify the features arising following adsorption. From this, we observe features at 2.45, 4.54, 7.51, 10.48, and 13.45 eV. These features are attributed to valence states of the adsorbate. The most interesting change following the adsorption of dopamine is the decrease in the intensity of the band gap state, which is seen as a small dip in the difference spectrum at around 1 eV and may arise from adsorption at oxygen vacancy sites. Along with the reduction in the Ti3þ peak in the Ti 2p spectrum and the reduction in the intensity of the band gap state following dopamine adsorption, binding energy shifts are observed in the Ti 2p, valence band, and O 1s spectra. The Ti 2p spectrum is shifted to lower binding energy by 0.2 eV, and the O 1s and valence band spectra are shifted, also to lower binding energy, by 0.3 eV following the adsorption of dopamine. These binding energy shifts may be explained by band bending at the anatase (101) surface due to the adsorption of dopamine from the residual vacuum, resulting in removal of Ti3þ at the surface. On the clean single crystal, the bands bend to higher energy because of the presence of Ti3þ at the surface, resulting in the formation of a depletion region near the surface caused by the excess carriers.30 Following the adsorption of dopamine, the bands are bent back the other way as Ti3þ is reduced to Ti4þ and the excess carriers are removed. A similar band-bending effect has been observed following the adsorption of bi-isonicotinic acid on a TiO2(110) rutile surface.31 The data seem to suggest that the band gap state is reduced in intensity more than the peak due to Ti3þ in the Ti 2p spectrum. The reason for this is not entirely clear but may be due in part to the presence of Ti3þ interstitials in the subsurface region as well as surface oxygen vacancies. Both types of Ti3þ species would contribute to the peak in the Ti 2p spectra32 whereas the band gap state is thought to be related only to surface oxygen vacancies.29 However, this still does not satisfactorily account for the intensity of the Ti3þ peak because the number of interstitials would have to be extremely large. Further investigation of this is of some fundamental interest but is beyond the scope of the current work. For the purposes of this article, it is sufficient to say that the intensities of this peak and the band gap state are reduced slightly following the adsorption of dopamine, which we attribute to the removal of surface oxygen vacancies following dopamine adsorption. Figure 4 shows the carbon K-edge NEXAFS spectra of dopamine on anatase TiO2 for incident radiation angles of 30-90° with respect to the surface. The spectra were normalized by dividing the dopamine-dosed TiO2 C K-edge NEXAFS spectrum by the (29) Yim, C. M.; Pang, C. L.; Thornton, G. Oxygen vacancy origin of the surface band-gap state of TiO2(110). Phys. Rev. Lett. 2010, 104, 3. (30) Diebold, U. The surface science of titanium dioxide. Surf. Sci. Rep. 2003, 48, 53–229. (31) Taylor, J. The Adsorption, Charge Transfer Dynamics and Kinetics of Organic Molecules at Surfaces. Ph.D. Thesis, The University of Nottingham, Nottingham, U.K., 2007. (32) Wendt, S.; Sprunger, P. T.; Lira, E.; Madsen, G. K. H.; Li, Z. S.; Hansen, J. O.; Matthiesen, J.; Blekinge-Rasmussen, A.; Laegsgaard, E.; Hammer, B.; Besenbacher, F. The role of interstitial sites in the Ti 3d defect state in the band gap of titania. Science 2008, 320, 1755–1759.
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Figure 4. Carbon K-edge NEXAFS spectra recorded from an ∼1 ML dopamine-dosed anatase TiO2(101) single crystal. Spectra are normalized by dividing the dosed spectra by spectra recorded from the clean surface and setting the step edge jump equal to unity. The inset shows the area of the πa* peaks (normalized to 1) vs the angle of the incident photon beam with respect to the surface. The solid line is a best fit to the data using St€ ohr equations.33 The error bars are estimated from the χ2 value of the fit to the individual NEXAFS spectra.
clean TiO2 C K-edge NEXAFS spectrum. The step edge at the ionization potential on the resulting spectrum was then set equal to unity. The peak at 284.6 eV arises from excitations from C 1s into the π* orbitals of the carbon atoms in the phenyl ring of the dopamine molecule that are not bonded to oxygen atoms, corresponding to carbon atoms 3-6 in Figure 1 (hereafter πa*). The peak at 285.9 eV originates from C 1s π* excitations of the carbon atoms in the phenyl ring that are bonded to oxygen atoms, corresponding to carbon atoms 1 and 2 in Figure 1 (hereafter πb*). These peaks have an area ratio of approximately 2:1 because there are four carbon atoms in the ring that are not bonded to oxygen and two that are. This is in agreement with the C 1s XPS data discussed earlier. Gaussians and a step edge were fitted to each spectrum to produce the best fit to the data (Supporting Information, Figure S1). The areas of the πa* peaks at each angle were calculated and plotted as a function of the angle of incidence as shown in the inset Langmuir 2010, 26(18), 14548–14555
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Figure 6. NEXAFS spectrum of dopamine on anatase TiO2(101) and a StoBe-generated NEXAFS spectrum of a dopamine molecule.
Figure 5. Schematic diagram of a dopamine molecule with the phenyl ring approximately normal to an anatase TiO2(101) surface. Red spheres represent O atoms, small white spheres represent H atoms, and light-gray spheres represent Ti atoms.
of Figure 4. The graph is normalized to the πa* peak intensity recorded with the incident beam 90° to the surface. St€ohr equations33 were fitted to the data points. The data point for 30° with respect to the surface was excluded from the fit because it appears to be anomalous compared to the other data points. This deviation is likely to occur because measurements taken at angles close to the sample surface may result in the photon beam interacting with dopamine on the sample holder because the beam has a larger footprint on the sample at these grazing incidence angles. The fit gives an angle for the π* orbital of 90 ( 5° away from the surface normal. The plane of the aromatic ring is therefore oriented roughly normal to the surface (i.e., the molecule is upright on the surface as shown schematically in Figure 5). Li et al.17 found the angle of catechol molecules on rutile TiO2(110) to be tilted by 15-30° from the surface normal. We suggest that this difference arises from steric effects associated with the amine side chain in the dopamine molecule. The NEXAFS spectrum in Figure 6, which is taken over a slightly larger photon energy range than those in Figure 4, contains a feature at around 281.5 eV photon energy. It was first thought this may be a normalization problem in dividing the dosed by the clean spectrum, arising from constant binding energy features from the substrate passing through the photon energy window, but calculations on this system and for catechol adsorption on rutile TiO2(110) and anatase TiO2(101)34 suggest another possible origin of this feature, which is discussed below. 3a. Computer Modeling. Figure 7 shows the HOMO and LUMO of the molecule as calculated by Gaussian, and Figure 6 shows the comparison between an experimental NEXAFS spectrum (33) St€ohr, J. NEXAFS Spectroscopy; Springer-Verlag: Berlin, 2003. (34) Thomas, A.; Syres, K.; Flavell, W.; Spencer, B.; Preobrajenski, A.; Bondino, F.; Malvestuto, M. To be submitted for publication.
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Figure 7. (a) HOMO of an isolated dopamine molecule. (b) LUMO of an isolated dopamine molecule.
of dopamine on anatase TiO2(101) (taken at normal incidence) and the StoBe-calculated NEXAFS spectrum of the isolated dopamine molecule. (The total StoBe NEXAFS was shifted slightly so that the πa* peak overlaps that of the experimental data.) StoBe does not calculate the separation between the two π* peaks to be the same as was measured experimentally. In addition, the StoBe-generated spectrum predicts the lowest-energy σ* features to occur at lower energy than observed experimentally. The difference between experiment and calculation is partly attributed to the fact that the calculation is on the isolated dopamine molecule and does not consider the effects of the bonding geometry or the interaction of the molecule with the anatase surface. Gaussian was then used to generate a geometry-optimized cluster of dopamine on the anatase TiO2(101) surface as shown in Figure 1. The phenyl ring of the adsorbed dopamine molecule is found to tilt at an angle of 48° from the anatase surface normal although experimentally the angle of the phenyl ring of the dopamine molecule was found to be oriented with the plane of the ring normal to the surface (i.e., the cluster calculation gives a much larger tilt angle from the surface normal than found experimentally). We attribute this difference to the fact that in the experimental data we have ∼1 ML of dopamine molecules on the anatase TiO2(101) surface that will interact with each other, causing them to “stand up” on the surface.33 Further work is DOI: 10.1021/la1016092
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Figure 9. Comparison of the experimental C K-edge NEXAFS spectrum of dopamine on anatase (blue line) with the StoBecalculated NEXAFS spectrum of dopamine on the anatase cluster shown in Figure 1 (black line). Figure 8. HOMO, LUMO, LUMO þ 1, LUMO þ 2, and LUMO
þ 6 of dopamine on an anatase TiO2(101)-optimized cluster as calculated by Gaussian.
planned to investigate the orientation of submonolayer coverages of dopamine on the anatase TiO2(101) surface. Figure 8 shows the HOMO, LUMO, LUMO þ 1, LUMO þ 2, and LUMO þ 6 of the dopamine on anatase TiO2(101) cluster calculated by Gaussian. The HOMO of the cluster is primarily located on the dopamine molecule and looks similar to the HOMO of the isolated dopamine molecule. The LUMO is primarily located on the TiO2 surface rather than on the dopamine molecule. LUMO þ 1 and LUMO þ 2 of the dopamine/ anatase TiO2(101) system are also more localized on the TiO2 surface than the dopamine molecule, although as can be seen in Figure 8 there are states on the carbon atoms and the on surface in LUMO þ 1 and LUMO þ 2. It is only when LUMO þ 6 is reached that the orbital density on the molecule is found to be similar to that of the free dopamine molecule. LUMO þ 1 and LUMO þ 2 are believed to be new states that are introduced into the system as a result of bonding between dopamine and the TiO2 surface. This is in agreement with a similar calculation on catechol on a hydrated Ti4þ ion in solution by Duncan et al.35 Figure 9 shows an experimental C K-edge NEXAFS spectrum of dopamine on anatase TiO2 (taken at normal incidence) and the StoBe-generated NEXAFS spectrum of dopamine on an anatase cluster. (The total NEXAFS spectrum was shifted slightly to overlap with the experimental data.) The separation of the π* resonances in the calculated spectrum agrees much better with the experimental data than with the calculation on the isolated dopamine molecule. However, the calculation, as with the isolated dopamine calculation, predicts the σ* resonances to occur at too low an energy. In the calculated spectrum, there are two peaks at binding energies of 281.7 and 282.8 eV that were not present in the calculated isolated dopamine molecule spectrum. In the experimental NEXAFS spectra shown in Figures 6 and 9, a feature is observed at a photon energy of 281.5 eV that coincides with the two calculated peaks at 281.7 and 282.8 eV. Similar peaks are (35) Duncan, W. R.; Prezhdo, O. V. Electronic structure and spectra of catechol and alizarin in the gas phase and attached to titanium. J. Phys. Chem. B 2005, 109, 365–373.
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observed in experimental and calculated NEXAFS spectra following the adsorption of pyrocatechol on rutile TiO2(110) and anatase TiO2(101) surfaces.34 These peaks are therefore tentatively assigned to LUMO þ 1 and LUMO þ 2 shown in Figure 8. These are the only states below LUMO þ 6 that are found to lie on the ring carbons, as required for an intra-atomic NEXAFS process. A comparison of LUMO þ 6 in the cluster to the LUMO of the unoccupied molecule suggests that this state contributes to the observed experimental π* peak at 284.6 eV. Although these cluster calculations seem to support our data, it should be noted that they have limitations. Indeed, as mentioned above, the cluster calculation results in a tilted molecular geometry whereas our experimental data shows the molecule to be in an upright orientation. Clearly this would have some effect on the distribution of the unoccupied molecular/surface states that may not be accounted for in these cluster calculations. Periodic DFT calculations may give a more accurate description of the system because they will account for the whole surface and take into account interactions between neighboring dopamine molecules. Figure 10 shows a schematic energy band diagram of the dopamine-TiO2 system using a combination of the experimental and calculated data. The position of the dopamine HOMO is taken from a thick film of dopamine adsorbed on rutile TiO2(110) (Supporting Information, Figure S2), and the separation between the LUMO and HOMO is set to 4.4 eV as measured using UV-vis absorption spectroscopy of dopamine in solution.36 The positions of the occupied states in TiO2 are taken from the valence band spectrum shown in Figure 3, as are those recorded from the dosed surface. The positions of the new states arising from LUMO þ 1 and LUMO þ 2 are taken from the experimental NEXAFS spectrum, as the center of the broad feature at around 283 eV, relative to LUMO þ 6 of the molecule. The position of the calculated ground-state energy of LUMO þ 1 is shown by a dashed line situated at around 0.6 eV below the LUMO þ 6 energy. It is stressed that this Figure is schematic, and we have made a number of assumptions. (The main one, in addition to those (36) Oni, J.; Westbroek, P.; Nyokong, T. Electrochemical behavior and detection of dopamine and ascorbic acid at an iron(II)tetrasulfophthalocyanine modified carbon paste microelectrode. Electroanalysis 2003, 15, 847–854. (37) Weinhold, M.; Soubatch, S.; Temirov, R.; Rohlfing, M.; Jastorff, B.; Tautz, F. S.; Doose, C. Structure and bonding of the multifunctional amino acid L-DOPA on Au(110). J. Phys. Chem. B 2006, 110, 23756–23769.
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because there is no clear evidence of overlap of states in the cluster calculations and NEXAFS is primarily an intra-atomic process these data do not unambiguously determine the charge-transfer method. Further work is underway to investigate the chargetransfer mechanism in more detail.
Figure 10. Schematic band energy diagram of the TiO2-dopamine surface. The dashed line immediately below LUMO þ 6 indicates the position of LUMO þ 1 in the ground-state cluster calculation. Further details of the construction of the diagram are described in the text.
already mentioned, is that the HOMO-LUMO þ 6 separation of the cluster is the same as that of the HOMO-LUMO separation of the free molecule, as is suggested by our calculations.) It can be seen that the positions of the new states observed in the NEXAFS spectra coincide with the bottom of the conduction band of TiO2. A simple calculation based on this Figure and using the assumptions mentioned above gives the separation between the HOMO of the molecule and LUMO þ 1/LUMO þ 2 to be around 2.6 eV, which is in reasonable agreement, with the 3 eV observed experimentally by UV-vis absorption.12 This means that the position of these states near the bottom of the conduction band could allow transfer to occur via an excited molecular state as seen in other dye molecules.4 In this scenario a photoexcited electron could be transferred to the new LUMO þ 1/LUMO þ 2 states and then injected into the conduction band, contrary to the direct mechanism suggested by Persson et al.12 However, because these states lie so close to the bottom of the conduction band it is not possible to say with certainty from these data that a direct molecular HOMO to TiO2 conduction band charge transfer does not occur. Indeed, if there is strong hybridization between these states and those of the substrate conduction band then transfer would effectively occur directly to the conduction band. However,
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4. Conclusions Photoemission results suggest that the dopamine molecule adsorbs on the surface in a bidentate geometry (i.e., bonding through both oxygen molecules), although it is not possible to say unambiguously whether this is to one (chelating) or two (bridging) surface Ti atoms. Shifts were measured in the photoemission spectra upon the adsorption of dopamine along with a decrease in the Ti3þ-derived peak in the Ti 2p spectrum and an attenuation of the band gap state in the valence band spectrum that corresponds to oxygen vacancies. This suggests the dopamine is filling in the oxygen vacancies on the surface and “unbending” bands at the surface, creating a shift in the binding energies of the photoemission peaks. Carbon K-edge NEXAFS spectra indicate that the dopamine ring is aligned normal ((5°) to the surface. Calculations of the molecular orbitals of a TiO2-dopamine cluster show that the HOMO is located on dopamine and the LUMO is located on the TiO2 surface. The LUMO þ 1 and LUMO þ 2 states appear to be located on both the molecule and the surface. These unoccupied states give rise to a small peak in the calculated NEXAFS spectra, which agrees well with a similar peak observed in the experimental data and may allow a direct dopamine to TiO2 photoinjection mechanism as previously proposed for the catechol-TiO2 system12 through the hybridization of molecular states with those of the surface. However, our data also supports a mechanism of electron excitation into the new states, followed by charge transfer to the TiO2 conduction band. Acknowledgment. This work was supported by EPSRC travel grant EP/H0020446/1, and beamtime at Elettra was funded by Sincrotrone Trieste S.CpA We are also grateful to Lars Petterson for assistance with StoBe-deMon and Petter Persson for his original anatase TiO2(101) surface cluster that was modified for this work. Supporting Information Available: Normalized C K-edge NEXAFS spectrum at 60° from the surface showing the step function and Gaussian peaks that result in the best fit (solid black line) to the data. A thick film of dopamine adsorbed on a rutile TiO2(110) surface recorded at 110 eV photon energy. This material is available free of charge via the Internet at http://pubs.acs.org.
DOI: 10.1021/la1016092
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