Comparing Surface Binding of the Maleic Anhydride Anchor Group on

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J. Phys. Chem. C 2010, 114, 15015–15020

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Comparing Surface Binding of the Maleic Anhydride Anchor Group on Single Crystalline Anatase TiO2 (101), (100), and (001) Surfaces E. M. J. Johansson,*,†,‡ S. Plogmaker,† L. E. Walle,§ R. Scho¨lin,† A. Borg,§ A. Sandell,† and H. Rensmo† Department of Physics and Material Science, Uppsala UniVersity, SE-75120, Uppsala, Sweden, Department of Physical and Analytical Chemistry, Uppsala UniVersity, SE-75105, Uppsala, Sweden, and Department of Physics, Norwegian UniVersity of Science and Technology, NO-7491, Trondheim, Norway ReceiVed: May 28, 2010; ReVised Manuscript ReceiVed: July 19, 2010

We report on the surface binding of the maleic anhydride (C4H2O3, MA) on the TiO2 anatase (101), (100), and (001) single crystal surfaces. The MA anchor group has recently been used for dye adsorption in solar cells based on nanostructured anatase, and the results reported here are partly discussed with respect to such systems. MA was deposited simultaneously onto the (101), (100), and (001) TiO2 single crystal surfaces in UHV, and the surface binding was investigated with electron spectroscopy. The O1s and C1s core-level spectra were compared to a multilayer of MA to investigate the differences in bonding to the anatase surfaces. The results suggest a surface chemistry where the molecule reacts and the MA ring opens when adsorbed at the (101) and (100) surfaces. The molecule anchors via four oxygen atoms, similar to bonding with two carboxylic groups on TiO2. For the (001) surface, the spectra indicated a different adsorption geometry. A small amount of electronic states in the bandgap of the TiO2 surfaces was observed both before and after MA was deposited onto the surfaces, and on the (101) and (100) surfaces, the intensity of these surface states was slightly enhanced after deposition of MA. Introduction TiO2 surfaces are intensively investigated because of the interesting properties of the oxide and its use in various applications including catalysis and photocatalysis,1-3 chemical sensors and biocompatibility,4-8 as well as for different energy applications.9,10 Consequently, many physical and chemical properties of different pristine and molecularly modified surfaces have been investigated.11 Many investigations focus on single crystal surfaces and in particular the behavior of the rutile TiO2(110).11 Although much of the commercial TiO2 catalysts used contain a large and most important portion of TiO2 with the anatase structure, much fewer studies have been performed on the anatase surfaces, since large single crystals are more difficult to obtain.12 Maleic anhydride (C4H2O3, MA) is a versatile material that with the assistance of oxide catalysts is used for the production of many different chemicals.13 In order to obtain a fundamental understanding of the decomposition and surface reactions of this molecule, a few model system studies on TiO2 surfaces have been performed.14 More generally, the surface reaction of maleic anhydride may be considered to have many similarities with carboxylic acids. Carboxylic acids are used in several applications anchoring organic molecules in order to functionalize TiO2. One such area is the use of dye molecules in nanostructured TiO2 based solar cells.9,15,16 Recently, a new set of purely organic dye molecules containing an anhydride anchoring moiety has been reported with interesting performance in solar cell configurations.17-19 It was suggested that the * To whom correspondence should be addressed. E-mail: [email protected]. † Department of Physics and Material Science, Uppsala University. ‡ Department of Physical and Analytical Chemistry, Uppsala University. § Norwegian University of Science and Technology.

anhydride moiety ring opens at the surface to form carboxylic bonds to the TiO2 surface.17 In the photocurrent generation process in dye-sensitized solar cells, photons are absorbed in a dye molecule, which is followed by a fast injection of electrons from the dye to the conduction band in the TiO2. How the dye molecules are attached to the TiO2 surface is therefore a very important and fundamental property. An important mechanism in the use of the dye molecules for this reason concerns the role and the chemistry of the anchor group. A fundamental understanding of this chemistry obtained from synchrotron based photoelectron spectroscopy is the target of the present work. Specifically, the investigation compares the adsorption of maleic anhydride at three different anatase surfaces (101), (001), and (100), in order to illuminate similarities and differences. These low-index anatase surfaces have low surface energies12 and are therefore frequently occurring in the TiO2 nanoparticles used in dyesensitized solar cells.20 The differences in the anchoring of the molecule to the different anatase surfaces present in the nanoparticles will, in particular, affect the electron transfer rates to the TiO2 in the solar cell and thereby the solar cell efficiency. Experimental Section The anatase TiO2 (101), (100), and (001) oriented single crystals and the maleic anhydride (C4H2O3, MA) were purchased from PI-KEM Ltd. and Sigma-Aldrich, respectively. The TiO2 crystals were new and were cleaned by cycles of Ar sputtering at 1 kV (about 20 min) followed by heating of the sample (770 K for 5-10 min) in O2 pressure of 10-6 mbar, with a background pressure of about 1 × 10-10 mbar. The annealing and oxygen pressure were stopped simultaneously, and the samples were thereafter cooled down in a vacuum. The surface structures of the crystals were confirmed by LEED. MA powder

10.1021/jp104897k  2010 American Chemical Society Published on Web 08/18/2010

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was placed in a small chamber connected to the sample chamber via a needle valve. The chamber with the MA powder was evacuated separately, and the vapor from the MA powder kept at room temperature was thereafter introduced into the sample chamber. In this report, the focus is to determine the interaction between the MA molecules and the different TiO2 surfaces. We have for this reason concentrated on preparing all of the TiO2 surfaces simultaneously, to obtain similar conditions when depositing MA. It is difficult to prepare the clean (101), (001), and (100) surfaces at the same time, since one cleaning procedure only must be selected for all three samples. After the preparations, we therefore have a small potassium and carbon contamination on the surfaces (about 5% of the signal in the C1s spectra), which we have subtracted from our C1s spectra. MA was deposited (at 5 × 10-7 mbar for 5 min at room temperature) and from the decrease in the Ti2p spectra (about 20%), and using a mean free path of about 0.7 nm,21 the coverages of MA are estimated to be in the submonolayer to monolayer regime, and we will hereafter refer to these as submonolayers. A multilayer of MA was deposited (7 × 10-8 mbar for 10 min) onto the surface cooled to about 100 K. All spectra were recorded at beamline D1011 of the Swedish National Synchrotron Facility MAX II.22 The endstation comprises a 200 mm radius hemispherical electron energy analyzer of Scienta type. For the TiO2 without MA and with submonolayers of MA, the binding energy (BE) calibration of the peaks versus the Fermi level was achieved by relating the energy scale of the sample to the Fermi level of a platinum foil in electric contact with the sample. For the multilayer, the peaks were energy calibrated using the C1s peak, which was aligned with C1s for the submonolayer coverage of molecules. No effects due to charging or X-ray damage were observed during the experiments. The intensities of the spectra of the submonolayers were calibrated using the current in the synchrotron ring, if not stated otherwise. The multilayer spectra were intensity calibrated to enable a simple comparison with the submonolayer spectra. The C1s X-ray absorption spectra were recorded using a multichannel plate for detection of secondary electrons in partial yield mode. The retardation voltage was set to 100 V, and the detector was placed orthogonally to the sample surface normal as well as to the polarization vector of the X-rays. This setup with detection of secondary electrons in partial yield mode makes the detection surface sensitive with a probe depth estimated to about 10 Å from the kinetic energy of the C Auger electrons. A background spectrum, obtained from the clean TiO2 substrate, was subtracted from the C1s XAS spectrum to avoid spectral features due to intensity variations at different energies of the photons reaching the sample. Results and Discussion As a basis, we first report the core levels of the pristine TiO2 surfaces before MA deposition and the C1s XAS of a multilayer of MA, which confirm the quality of the surfaces and the deposition method, respectively. The Ti2p and O1s spectra for the (101), (001), and (100) surfaces are shown in Figures 1 and 2, respectively. The peaks at 458 and 465 eV in Figure 1 correspond to Ti2p3/2 and Ti2p1/2 from the Ti4+ in TiO2, and we do not observe any contributions from other oxidation states of titanium within the noise limit of the experiment. This means that the number of Ti3+ defect states is well below a few percent for these surfaces. In these surface sensitive measurements measured with a photon energy of 610 eV, the Ti2p spectrum of the (101) surface was found to be slightly more intense than that for the other surfaces. It indicates a higher concentration

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Figure 1. Ti2p spectra of the clean anatase (101), (100), and (001) crystal surfaces measured with a photon energy of 610 eV.

Figure 2. O1s spectra of the clean anatase (101), (100), and (001) crystal surfaces measured with a photon energy of 610 eV.

of Ti atoms in the outermost surface of this crystal surface compared to the other crystal surfaces. In the O1s spectra of the different TiO2 surfaces in Figure 2, only one peak corresponding to the TiO2 oxygen is observed in the spectra. The cleaning procedure is therefore found to be effective. The C1s XAS spectrum of a multilayer of MA deposited onto the TiO2 surfaces is shown in Figure 3. The spectrum agrees with previous results reported for C1s XAS from a multilayer of MA,23 confirming that the deposition of MA is of high quality. MA was deposited onto the three cleaned TiO2 surfaces simultaneously under the same experimental conditions (5 min of MA deposition with a pressure of 5 × 10-7 mbar at 300 K, i.e., 150 L), yielding a submonolayer coverage of MA. O1s spectra of the molecules on the three TiO2 surfaces were measured, and the difference spectra obtained from these spectra after subtraction of the O1s spectra of the clean surfaces are shown in Figure 4. Also, the O1s spectrum of a multilayer of MA is included in the figure. The multilayer spectrum shows two contributions with an intensity ratio of 2:1, which agrees

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Figure 3. C1s XAS for a multilayer of MA.

Figure 4. O1s difference spectra obtained from subtracting the O1s spectra of the clean anatase surfaces from the O1s spectra for MA on the anatase surfaces.

with the two chemically different oxygen atoms (two (dO) and one (sOs)) in the molecule, as previously reported for a multilayer of MA on ZnO.24 In the difference spectra of MA on the (101) and (100) surfaces, only one peak can be observed and the fwhm of this peak is 1.5 eV, which is very close to the value (1.4 eV) for the peaks observed in the multilayer case. This shows that there is only one type of oxygen for MA deposited on the (101) and (100) surfaces and we can therefore conclude that all oxygen atoms bind with a similar configuration to the TiO2 surface. This is a situation very similar to what is

found for dye anchoring molecules with carboxylic acid binding groups on TiO2 rutile (110)25,26 and anatase.27 The results therefore suggest that the MA ring opens up and forms two carboxylic bonds to the anatase (101) and (100) surfaces, which result in chemically similar oxygen atoms in the adsorbed species. On the basis of the change in the UV-vis absorption spectrum when adsorbed to TiO2 nanoparticles,17 such ring opening has also been suggested for larger dye molecules containing an anhydride attachment group. The two carboxylic groups formed by the ring opening probably bind in a bidentate configuration to two Ti atoms, which have previously been suggested for dye attachment molecules with carboxylic acid anchoring groups25-27 and also for formic acid on rutile (110) (see, e.g., refs 28 and 29) and anatase (001) (see, e.g., ref 30). A monodentate configuration would result in inequivalent oxygen atoms, which is more unlikely, since we observe only one oxygen peak in the O1s spectra. A binding through ring opening has also been suggested for MA adsorbed to rutile TiO2(001).14 When the MA molecule ring opens and forms the two bidentate carboxylic bonds to the surface, one extra oxygen atom originating from the TiO2 surface is required, since there are only three oxygen atoms in the MA molecule itself. The (101) surface is the most common surface of the anatase nanoparticles used in energy applications,20 and we focus on a model of the adsorption geometry on this surface, which is shown in Figure 5. The TiO2 surface in the model is constructed from the bulk geometry, terminated at the (101) surface without any reconstruction, since the (101) surface structure is rather similar to a bulk terminated structure.12 In the model, the MA molecule is ring opened and the carbon-carbon bond lengths kept at the same values as in the free molecule. The model described in the figure shows that it is possible to find an adsorption geometry of the molecule that agrees with the obtained XPS results. The oxygen atoms originating from the MA molecule are highlighted in yellow, and the oxygen atom originating from the TiO2 surface but interacting with the molecule is highlighted in green. The three oxygen atoms originating from MA are attached to the 5-fold coordinated Ti atoms, Ti(5), on the surface, whereas the fourth oxygen bond to MA originates from the TiO2 surface. We may estimate the coverage of oxygen atoms originating from MA on the surface by comparing the O1s spectra observed for MA on the anatase (101) surface with H2O deposited on

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Figure 6. C1s spectra for a multilayer of MA and MA submonolayers on the anatase (101), (100), and (001) crystal surfaces measured with a photon energy of 390 eV.

Figure 5. Proposed model for MA adsorption on the (101) surface. The oxygen atoms originating from MA are highlighted in yellow, whereas the oxygen atom binding to MA originating from the TiO2 surface is highlighted in green. Part a monitors the model from a side perspective and part b from above.

TiO2 anatase and rutile surfaces.31 The amount of oxygen from adsorption of H2O on rutile (110) is extensively studied,31 and from this comparison we can estimate the amount of oxygen relative to the number of 5-fold coordinated Ti atoms, Ti(5), on the surface. The intensity of the peak corresponding to oxygen bound to MA (including both the oxygen atoms originating from MA and the oxygen atoms from TiO2 bound to MA) was compared to the intensity of the peak corresponding to H2O in an O1s spectrum with water deposited on TiO2. From this comparison, we could estimate that the amount of oxygen bound to MA is 1.4 times higher than the amount of 5-fold coordinated Ti atoms, Ti(5), on the surface. In the binding model described above, one of the four oxygen atoms bound to each MA molecule originates from the TiO2 surface. For a surface completely covered with MA molecules, the model therefore results in an amount of oxygen which is 1.33 times higher compared to the amount of Ti(5) atoms. This agrees rather well with the value of 1.4 estimated from the experiments, thus supporting the model for binding of the MA molecules on the (101) surface suggested above. For the (001) surface, the O1s difference spectrum shows two peaks. The adsorption of MA at this surface therefore differs from that on the (101) and (100) surfaces, revealing oxygen atoms in a rather different chemical state. The energy difference between the two peaks is about 2.1 eV, which is larger than that in the multilayer (1.3 eV). One explanation may be that the MA molecules on this surface do not bind by the ringopening reaction, that was suggested for the (101) and (100) surfaces, but instead bind with the oxygen atoms directly to the titanium atoms present on the surface, without ring opening. Although a ring opening cannot be excluded, it is difficult to find a model with ring opening that explains the origin of the high binding energy oxygen peak. The peak at high binding

energy is also rather broad (2.2 eV), which suggests that there is a mixture of binding configurations on the (001) surface. The clean (001) surface of anatase TiO2 undergoes a (4 × 1) reconstruction in UHV.32,33 From scanning tunneling microscopy (STM) results in conjunction with DFT calculations, a structure model of the TiO2(001)-(4 × 1) surface has been derived.34,35 The surface can be described as consisting of ridges, made up by TiO3 chains with 4-fold coordinated Ti atoms, separated by terraces with 5-fold coordinated Ti atoms. Therefore, the ridges have been found to be very reactive upon adsorption of molecules.36,37 This specific property of the (001) surface compared to the (101) and (100) surfaces may be part of the explanation for a difference in the adsorption structure of the MA molecule on the (001) surface compared to the (101) and (100) surfaces. Specifically, it is most likely that the 4-fold coordinated Ti atoms present on the (001) surface affect the adsorption configuration of MA differently than the 5-fold coordinated Ti atoms on the (101) and (100) surfaces. The C1s spectra for the submonolayers of MA on the surfaces and a multilayer of MA are shown in Figure 6. Two peaks are observed, corresponding to olefinic and carbonyl carbon atoms, at 286 and 289 eV, respectively.24 The intensities of the two peaks are similar, which agree with the stoichiometry of the molecule. The binding energy difference between the peaks is slightly smaller (0.2 eV) for the submonolayers in comparison with the multilayer, which shows that the carbon atoms are rather unaffected by the binding of the molecule to the surface. Comparing the intensities of the total C1s spectra for the different surfaces with MA, we observe that they are rather similar: 1.0:1.0:0.9 for the (101), (100), (001) surfaces, respectively, implying that the coverage of molecules is rather similar on the three surfaces. In Figure 7, two C1s spectra from the submonolayer on the (101) surface measured with different surface sensitivities are shown. The most surface sensitive measurement is performed with lower photon energy and with a grazing emission angle. In the most surface sensitive measurement, we observe that the peak corresponding to the olefinic carbon is enhanced. This surface sensitive experiment suggests that this carbon is located outside the carbonyl carbon, which agrees with the adsorption

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Figure 7. C1s spectra of a submonolayer of MA on the (101) surface measured at two different photon energies (390 and 365 eV) and electron emission angles (90 and 10°) to obtain different surface sensitivities.

geometry suggested from the O1s spectra, with the oxygen binding to the TiO2(101) surface. Diffraction effects on the emitted electrons may also affect the intensities of the C1s signals. However, the agreement between the multilayer measurement and the less surface sensitive measurement on the submonolayer indicates that these effects are small. In addition, the less surface sensitive measurement on the submonolayer and the measurement on the multilayer agree with the stoichiometry for the molecule as mentioned above, which indicates that the diffraction effects are negligible on these samples. From the measurements with different surface sensitivities, we thus conclude that the molecule stands upright on the surface, with the oxygen atoms binding to TiO2, and exclude the possibility of the MA ring lying down on the surface. Discussing the results from a solar cell perspective, the suggested molecular structure of MA on the (101) and (100) surfaces is favorable in several aspects. First, the carboxylic binding is rather strong, which allows for a chemically stable system and therefore stable solar cells. Second, the orientation of the MA on the surface suggests that also a complete dye molecule with this anchor group will preferentially stand upright on the surface and bind with the anhydride anchoring group. This will enable injection of excited electrons into TiO2 from the dye molecule, since the lowest unoccupied molecular orbitals of the dye molecules used are located on the part of the molecule including the anhydride group and hence close to the TiO2.17,19 Also, this orientation will reduce recombination of the resulting hole in the dye molecule after electron injection, since the HOMO level in the dye is located on the part furthest from the anhydride anchoring group and thereby furthest away from the TiO2 surface.17,19 Third, the results confirm the binding chemistry at the surface which for conjugated/aromatic dyes will have large effects on the frontier electronic structure.17 On the (001) surface, however, the O1s spectra indicated a different bonding of MA to the substrate. It is therefore possible that the dye molecule on the (001) surface binds differently, which may result in a molecular dye orientation that instead limits electron injection and enhances recombination of the photogenerated charges. The valence electronic structure with and without MA is shown in Figure 8. An overview of the valence region for the

Figure 8. (a) Overview of the valence spectra for the clean (101) surface and the (101) surface with a submonolayer of MA. (b) Magnification of the low binding energy region in the valence spectra of the anatase (101), (100), and (001) crystal surfaces and these crystal surfaces with a submonolayer of MA. All spectra were measured at a photon energy of 130 eV.

(101) surface is shown in Figure 8a, and a magnification of the outermost structure of the valence electronic structure of the (101), (100), and (001) surfaces with and without MA is shown in Figure 8b. The spectra are intensity calibrated versus Ti3p, to compare differences related to changes in the substrate defect states. Defect states in this binding energy region are often referred to as bandgap states, since the states are located between the valence and conduction band. If we first consider the surfaces without MA, we observe that the cleaning/annealing procedure used here gives rise to clear differences in the amount and energy distribution of such bandgap states. We observe states at a binding energy of about 1-2 eV below the Fermi level. At the (101) surface, we measure the highest intensity from bandgap states, while at the (001) surface we measure the lowest intensity from these states. This may be expected, since the (101) surface is more prone to defect formation than the (001) surface.38,39 It is clear that the amount of such states, generally considered to originate from Ti atoms with a strong Ti 3d character, is low. The detailed explanation of the bandgap

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states of TiO2 has been investigated previously40-44 and is not within the scope of the present paper. However, it is noteworthy that bandgap states are still present after the adsorption of MA and in Figure 8b we observe that the relative intensity of the bandgap states on the (101) and (100) surfaces increases slightly upon surface adsorption. The presence of such bandgap states is generally considered important for the electronic transport properties of solar cells based on anatase nanoparticles, and our results indicate that the adsorption of molecules at the surface may increase the number of such states. On the (001) surface, on the other hand, there is instead a small decrease of the bandgap states. Conclusions MA deposited onto the TiO2(101), (100), and (001) single crystal surfaces in UHV was studied with photoelectron spectroscopy. In the O1s spectra, only one contribution was observed from MA on the (101) and (100) surfaces compared to the two O1s peaks observed for the MA multilayer. These results suggest that the MA ring opens upon adsorption at the (101) and (100) surfaces and anchors with four similar oxygen atoms, similar to two carboxylic groups on TiO2. In contrast, the O1s spectrum for MA on the (001) surface displayed two oxygen peaks, indicating a different adsorption geometry possibly without the ring-opening reaction. Comparing two C1s spectra from MA on the (101) surface, measured with different surface sensitivities, supports that MA stands upright with the oxygen atoms closest to the TiO2 surface, as expected from the O1s spectrum. Electronic states were observed in the bandgap region of TiO2 both before and after deposition of MA on the surfaces. The intensity of the electronic states in the bandgap of the (101) and (100) TiO2 surfaces increased slightly after MA deposition onto the surfaces. Acknowledgment. This work was supported by the Swedish Research Council (VR), the Go¨ran Gustafsson Foundation, the Carl Trygger Foundation, the Swedish Foundation for Strategic Research (SSF), and the Knut and Alice Wallenberg Foundation. L.E.W. has been supported through the Strategic Area Materials at NTNU. We thank the staff at MAX-lab for competent and friendly assistance. References and Notes (1) Hashimoto, K.; Irie, H.; Fujishima, A. Jpn. J. Appl. Phys. 2005, 44, 8269. (2) Rodriguez, J. A.; Jirsak, T.; Liu, G.; Dvorak, J.; Maiti, A. J. Am. Chem. Soc. 2001, 123, 9597. (3) Hoffmann, M. R.; Martin, S. T.; Choi, W. Y.; Bahnemann, D. W. Chem. ReV. 1995, 95, 69. (4) Zuruzi, A. S.; Kolmakov, A.; MacDonald, N. C.; Moskovits, M. Appl. Phys. Lett. 2006, 88, 102904. (5) Zhu, Y. F.; Shi, J. J.; Zhang, Z. Y.; Zhang, C.; Zhang, X. R. Anal. Chem. 2002, 74, 120. (6) Klinger, M. M.; Rahemtulla, F. R.; Prince, C. W.; Lucas, L. C.; Lemons, J. E. Crit. ReV. Oral Biol. Med. 1998, 9, 449. (7) MacDonald, D. E.; Deo, N.; Markovic, B.; Stranick, M.; Somasundaran, P. Biomaterials 2002, 23, 1269. (8) Cacciafesta, P.; Humphris, A. D. L.; Jandt, K. D.; Miles, M. J. Langmuir 2000, 16, 8167. (9) O’Regan, B.; Gra¨tzel, M. Nature 1991, 353, 737.

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