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J. Phys. Chem. B 2002, 106, 10102-10107
PES Studies of Ru(dcbpyH2)2(NCS)2 Adsorption on Nanostructured ZnO for Solar Cell Applications Karin Westermark, Håkan Rensmo, and Hans Siegbahn* Department of Physics, UniVersity of Uppsala, Box 530, S-751 21 Uppsala, Sweden
Karin Keis and Anders Hagfeldt Department of Physical Chemistry, UniVersity of Uppsala, Box 532, S-751 21 Uppsala, Sweden
Lars Ojama1 e and Petter Persson Department of Quantum Chemistry, UniVersity of Uppsala, Box 518, S-751 21 Uppsala, Sweden ReceiVed: NoVember 15, 2001; In Final Form: May 23, 2002
The interaction between the dye cis-bis(4,4′-dicarboxy-2,2′-bipyridine)-bis(isothiocyanato)-ruthenium(II), Ru(dcbpyH2)2(NCS)2, and nanostructured ZnO was investigated by photoelectron spectroscopy (PES) using synchrotron radiation. The results are compared with those of nanostructured TiO2 sensitized with the same dye, which to date is the most efficient system for dye-sensitized photoelectrochemical solar cells. When comparing the two metal oxides, differences in the surface molecular structure were observed both for low and high dye coverages, as seen by comparing the oxygen, nitrogen and sulfur signals. The origin of these differences is discussed in terms of substrate-induced dye aggregation and in variations in surface bonding geometries. The measurements also provide information concerning the energy matching between the orbitals of the dye and the ZnO valence band, which is of importance in photoinduced charge transfer.
Introduction Dye-sensitized nanostructured metal oxide film electrodes are intensively investigated today due to their potential use in photoelectrochemical (PEC) solar cells.1-3 One possible advantage with dye-sensitized PEC solar cells is the flexibility in the choice of material, in which all included components can be varied and their properties may be fine-tuned. Systems based on the dye cis-bis(4,4′-dicarboxy-2,2′-bipyridine)-bis(isothiocyanato)-ruthenium(II), Ru(dcbpyH2)2(NCS)2, adsorbed onto nanostructured TiO2 are the most extensively studied and, at present, the most efficient for photoelectrochemical solar cells.1,4,5 Attempts have been made to use ZnO in PEC solar cells;6-10 however, cells based on nanostructured ZnO have shown lower efficiencies compared to cells based on TiO2. Several possible explanations have been suggested in the literature, including poor light harvesting efficiency and weak electronic coupling between the excited dye and the ZnO conduction band for the ZnO-based system compared to the TiO2-based system.7,8,11 Recently, the formation of Zn2+/dye aggregates at the surface of the ZnO nanoparticles was discovered when using the Ru(dcbpyH2)2(NCS)2 dye. The aggregates fill the pores of the film and thereby result in lowered solar cell efficiencies.12 It was suggested that aggregation occurs due to the dissolution of zinc surface ions in which protons from the carboxylic acid groups of the dye are involved. Changes in parameters of the sensitization process, such as dye composition, concentration, and the residence time of the nanostructured film in the dye solution, had large effects on the photoelectrochemical output * To whom correspondence should be addressed. E-mail: hans.siegbahn@ fysik.uu.se.
of ZnO-based solar cells. Therefore, by optimizing the dye sensitization process one may find ZnO to be a candidate as a nanostructured material in efficient dye-sensitized solar cells.10 In this paper the adsorption of Ru(dcbpyH2)2(NCS)2 on nanostructured ZnO and the formation of Zn2+/dye aggregates are studied in more detail by photoelectron spectroscopy (PES) in order to shed further light on the possible causes for the different behavior of ZnO and TiO2 as materials in dyesensitized solar cells. PES provides possibilities to study both energy level alignment and chemical transformation of the dye at the dye/metal oxide interface. Experimental Section Preparation of Metal Oxide Electrodes. A colloidal ZnO solution was prepared by precipitation of ZnO powder by heating 0.1 M zinc nitrate aqueous solution (pH 5) in the presence of 0.1 M triethanolamine.13 The synthesis was conducted in tightly stoppered flasks, unstirred, at 100 °C for 24 h. The resulting suspension was then centrifuged. The precipitate was thoroughly washed with distilled water to remove any residual salt and then dried at 100 °C. All chemicals used were reagent grade; the water used was Milli-Q (Millipore Corporation). Dispersions of the ZnO powders were prepared by grinding a powder and water/acetylacetone mixture in a mortar and finally adding a small amount of Triton X-100. Colloidal TiO2 solution (Ti-nanoxide T) was purchased from Solaronix S. A., Aubonne, Switzerland.4 The electrodes were prepared by deposition of colloidal solution onto transparent conducting glass sheets (Libbey Owens Ford, fluorine-doped SnO2 glass with a sheet resistance of 8 Ω/square). Using Scotch tape as a frame and spacer, a layer
10.1021/jp0142177 CCC: $22.00 © 2002 American Chemical Society Published on Web 09/07/2002
Adsorption of Dyes on Nanostructured ZnO
Figure 1. SEM images of nanostructured ZnO sensitized for (a) 30 min and (b) 6 days.
was formed by raking off the excess of the colloidal solution with a glass rod. The electrodes were sintered at 450 °C for 30 min in airflow to form nanostructured film electrodes. The film thicknesses of the ZnO and TiO2 films were 10 and 2 µm, respectively, as determined by a Dektak 3 profilometer. The structural and photoelectrochemical characterization of the prepared ZnO electrodes is described elsewhere.13 Dye Sensitization. The ZnO electrodes used in the PES measurements were sensitized in 0.5 mM Ru(dcbpyH2)2(NCS)2 solution (Ruthenium 535 purchased from Solaronix S. A., Aubonne, Switzerland, in ethanol, Kemetyl, 99.6% spectroscopic quality) for 25 min, 60 min, and 5.5 h, respectively. The electrodes were immersed in room-temperature dye solution while still warm after the sintering process, typically 80 °C. The surface morphology of the prepared nanostructured ZnO films was investigated using scanning electron microscopy (SEM, LEO, 1530 Gemini). The SEM image of a ZnO film dye-sensitized for 25 min shows the structure of 150 nm sized ZnO particles forming a porous film (Figure 1a). No difference is observed when comparing this image with that of a plain ZnO film. For longer times (days) in the dye bath, the mechanical stability of the film is decreased. The SEM picture of such a film shows Zn2+/dye aggregates filling the nanoporous structure (Figure 1b). During the present study we noted that the nanostructured ZnO films used were strongly red colored on the outer part (electrode-solution interface) whereas they remained white in the zone close to the back-contact (electrode-substrate interface) even for the longest sensitizing times. A Ru complex related to Ru(dcbpyH2)2(NCS)2, Ru(bpy)2(dcbpyH2)‚2PF6-, adsorbed on a ZnO film was observed not to follow the same behavior. For
J. Phys. Chem. B, Vol. 106, No. 39, 2002 10103 the latter dye, the film was colored all the way through the nanostructured ZnO film within 25 min.14 The dye concentration (0.5 mM in ethanol), the temperature of the dye bath, and the film thickness were similar in both cases. This finding supports the results described in ref 12, where infrared and Raman spectroscopy together with electrochemical methods were used to follow the aggregation. It was shown that the formation of Zn2+/dye complexes is suppressed when the number of carboxyl groups in the dye is lowered, i.e., at higher local pH. PES measurements were also performed on precipitated Zn2+/dye complexes formed by mixing an ethanolic solution of Zn2+ ions (10 mL 0.5 M ZnCl2 in ethanol) and Ru(dcbpyH2)2(NCS)2-dye solution (50 mL 0.5 mM in ethanol) in a beaker. The solution was stirred for 1 h in room temperature. The precipitate formed was centrifuged and washed thoroughly with ethanol in order to remove remaining ZnCl2 and dissolved Ru(dcbpyH2)2(NCS)2. The samples for the PES measurements of such precipitated Zn2+/dye complexes, as well as samples of the Ru(dcbpyH2)2(NCS)2 dye powder were prepared by smearing the dye crystallites onto roughened gold substrates. This preparation was found to yield high-quality spectra without sample charging effects. The dye-sensitized TiO2 electrodes were prepared in a similar way to the ZnO electrodes: the electrodes were immersed in a 0.5 mM ethanolic Ru(dcbpyH2)2(NCS)2 solution after cooling to about 80 °C after the sintering process. PES Measurements. Photoelectron spectra were obtained using synchrotron radiation at beamline I411 of the Swedish National Synchrotron Source MAX.15,16 All spectra are energy calibrated with respect to the N1s peak of the bipyridine ligands in the Ru(dcbpyH2)2(NCS)2 dye. Repeated scans on the same spot illuminated for different times were recorded in order to follow a possible radiation damage process. No changes in peak shapes were detected, showing that no such damage process occurred within the time scale of the measurement. Results and Discussion Sensitization Procedure: High Dye Coverage. Carboxyl groups are common linkers for chemisorption of dye molecules onto oxide surfaces in dye-sensitized solar cells.17-20 For TiO2, the strong adsorption of the carboxyl group to the surface favors monolayer growth.4,21 For ZnO, on the other hand, the dye sensitization process has been shown to be more complex and the surface structure of the adsorbed layer depends on parameters such as sensitizing time and dye concentration. Thus, as apparent from Figure 1, unless these parameters are carefully controlled Zn2+/dye aggregates will form, reducing the solar-to-electric energy conversion efficiencies of Ru(dcbpyH2)2(NCS)2-sensitized nanostructured ZnO solar cells.12 To study the aggregation process and to find a preparation with a high dye loading having a minimum of dye aggregates, corresponding to the ideal case with respect to solar cell performance, we compared the O1s and Zn3d spectra for three different sensitizing times. It should be noted that PES is a surface sensitive measurement, and that the dye aggregation starts from the outermost surface of the nanostructured film where the dye concentration is highest. Therefore, it is expected that aggregates appear in a photoelectron spectrum for shorter sensitizing times than those needed for an optimally working solar cell. In Figure 2 the Zn3d spectra for ZnO sensitized in 0.5 mM ethanolic Ru(dcbpyH2)2(NCS)2 solution for different times are shown together with the Zn3d spectrum for dye aggregates
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Figure 2. Zn3d spectra of nanostructured ZnO sensitized for (a) 25 min, (b) 60 min, and (c) 5.5 h. (d) Zn3d spectrum of Zn2+/dye aggregates formed by precipitation in a Zn2+/dye solution. (hν ) 454 eV, binding energy calibration with respect to the bipyridine N1s signal.)
Figure 4. The O1s spectra for (a) a ZnO film sensitized 25 min and (b) a sensitized TiO2 film (solid lines). The dotted curves under each spectrum show the difference spectra obtained after subtracting the corresponding O1s substrate signal (measured for a plain film and intensity normalized versus the bulk substrate peak) and the curve fits for these (dashed and thin solid lines). (c) O1s spectrum of powder Ru(dcbpyH2)2(NCS)2, resolved into its dO and -OH components, at lower and higher binding energy, respectively. (hν ) 758 eV in all cases, binding energy calibration with respect to the bipyridine N1s signal.) Figure 3. O1s core level spectra of a plain, unsensitized nanostructured ZnO film and nanostructured ZnO films sensitized with Ru(dcbpyH2)2(NCS)2 for different times, indicated in the Figure. (hν ) 758 eV, binding energy calibration with respect to the bipyridine N1s signal. The corresponding Ru3d signals were found not to shift with respect to the N1s signals, and were in all cases single peaks.)
formed by precipitation in an ethanolic ZnCl2/dye solution. The presence of a Zn3d peak in the spectrum of the precipitate as well as the absence of a Cl2p signal (not shown) provides direct evidence that Zn ions bind to the dye molecules and form the precipitated Zn2+/dye aggregates in solution. For the sensitized nanostructured films, a shift toward the position of the Zn3d signal of the precipitated aggregates is observed with increasing sensitizing time. This indicates that a similar process occurs on the film surface as in forming the precipitated aggregates. In Figure 3 the O1s signals for the different sensitizing times are shown together with that of unsensitized ZnO. The O1s spectrum for the unsensitized ZnO electrode contains two peaks of similar intensities, shifted 1.3 eV with respect to each other. By varying the photon energy used in a PES measurement, the kinetic energy and thereby the escape depth of the emitted electrons is changed. The surface sensitivity in the measurement can therefore be tuned, and by using this method it was concluded that the O1s peak at lower binding energy (530.4 eV) originates from bulk oxygens, and the higher binding energy peak (531.4 eV) from surface oxygens. The surface oxygens
may correspond to OH- and H2O adsorbed on the surface, indicating the prescence of hydrated oxides.22,23 For the dye-sensitized nanostructured ZnO films, the O1s signal at the position of ZnO bulk oxygen decreases with increasing sensitizing time and the spectrum becomes more and more dominated by a peak at higher binding energy (531.8 eV). The main O1s peak from the dye appears at about the same binding energy as the ZnO surface oxygen, making it difficult to distinguish them from each other. For longer sensitizing times, however, the bulk oxygen signal almost disappears, indicating that a complete Zn2+/dye aggregate layer is formed and that the O1s spectrum is dominated by carboxyl oxygens in the aggregates. The O1s spectrum of this Zn2+/dye aggregatecovered surface is clearly different from that of the Ru(dcbpyH2)2(NCS)2 powder (Figure 4c), in that the O1s signal at higher binding energy, corresponding to carboxyl -OH, is significantly lower for the aggregates. This shows that the carboxyl groups are involved in forming the aggregates. However, as will be discussed further below, effects observed in the S2p spectrum indicate that also the NCS groups are influenced by the dye aggregation. The O1s spectrum of the Zn2+/dye aggregate layer has a small tail toward higher binding energies, probably originating from protonated carboxylic acid groups in the dye (see below). The presence of small amounts of protonated groups in the aggregates is in accordance with IR measurements12 and may
Adsorption of Dyes on Nanostructured ZnO reflect steric hindrances in the formation of Zn2+/dye aggregates. Since the dye molecules are large, it is probably impossible for all carboxyl groups of the dye molecules to participate in the formation of aggregates. Taken together, the spectral features discussed above support the view that for longer sensitizing times the formation of Zn2+/ dye complexes occurs at the ZnO surface and that the carboxyl groups play an active part in this formation. Sensitizing Procedure: Low Dye Coverage. In Figure 4a,b, the O1s spectra for ZnO and TiO2 sensitized in 0.5 mM ethanolic Ru(dcbpyH2)2(NCS)2 solution are shown. The spectrum shown for ZnO is that obtained for the shortest sensitizing time of 25 min. These two O1s spectra were analyzed in more detail in order to obtain a better understanding of the dye coverage on the ZnO surface in this low dye coverage case. In Figure 4a,b also difference spectra highlighting the contributions from the dye oxygens are shown for Ru(dcbpyH2)2(NCS)2 adsorbed onto ZnO as well as those for Ru(dcbpyH2)2(NCS)2 adsorbed onto TiO2. For the sensitized TiO2 the dye coverage is close to a monolayer.4,21 The difference spectra were obtained by subtracting a substrate peak (including surface and bulk oxygen) from the dye-sensitized ZnO and TiO2 spectra, respectively (normalization vs the metal oxide substrate bulk peak). The intensity relations between the dye and metal oxide oxygen signals were found to be 1:3.2 for Ru(dcbpyH2)2(NCS)2 on ZnO and 1:3.8 for Ru(dcbpyH2)2(NCS)2 on TiO2. Taking the stoichiometry of the oxide surfaces into account, these ratios indicate a similar or lower amount of dye on the ZnO surface than on the TiO2 surface. This preparation of the ZnO film was therefore used in the present PES study as a representative surface layer configuration for an efficient dye-sensitized nanostructured ZnO solar cell. Below we compare the results obtained from this preparation with results obtained from dyesensitized TiO2 in order to further elucidate similarities and differences between the binding as well as the electronic structure of Ru(dcbpyH2)2(NCS)2 on TiO2 and on ZnO. O1s. The O1s spectrum for the nonbonded Ru(dcbpyH2)2(NCS)2 molecule (dye powder, Figure 4c) contains two peaks with the same intensity, corresponding to the two nonequivalent oxygen atoms in the carboxyl groups. The higher binding energy peak (531.8 eV) represents the protonated oxygen atoms while the lower binding energy peak (533.2 eV) represents the doubly bonded oxygen atoms.24-27 The above result is similar to that found for nonbonded dcbpyH2.27 When the carboxylated dcbpyH2 molecules bind to a rutile TiO2 (110) surface, the carboxyl groups are deprotonated and bind to the suface in a bridge bonding mode.26,27 This means that both oxygens in the carboxyl group become chemically equivalent and appear as a single peak in the PES spectrum. The position of this peak is very close to that of the doubly bonded O1s peak in the nonbonded molecule, and is shifted 1.6 eV versus the main TiO2 substrate peak. In view of these findings, the difference spectra obtained for Ru(dcbpyH2)2(NCS)2 on ZnO and TiO2 (Figure 4a,b, cf. text above) are discussed as follows: The difference spectrum for Ru(dcbpyH2)2(NCS)2 adsorbed onto TiO2 (Figure 4b) can be deconvoluted into two peaks, having an intensity ratio clearly different from that of the nonbonded molecule (Figure 4c). The two peaks are interpreted as the protonated oxygens (the higher binding energy peak) and the doubly bonded oxygens together with the surface bonded oxygens (the lower binding energy peak) as discussed above. The intensity relation between the two peaks is found to be
J. Phys. Chem. B, Vol. 106, No. 39, 2002 10105
Figure 5. S2p core level spectra of (a) Ru(dcbpyH2)2(NCS)2 on ZnO (b) Ru(dcbpyH2)2(NCS)2 on TiO2 (hν ) 454 eV). The binding energy was calibrated with respect to the bipyridine N1s signal.
1:3, as expected if the dye molecule binds to TiO2 via two out of the four carboxyl groups of the complex in a bridge bonding mode, similar to the dcbpyH2 molecule adsorbed on rutile. The O1s difference spectrum for Ru(dcbpyH2)2(NCS)2 adsorbed on ZnO, however, displays a different shape from that of the dye adsorbed on TiO2. The intensity relation between protonated oxygens and the signal at the position of bridging oxygens is 1:2.2 in this case. The O1s difference spectrum obtained for a related dye, Ru(bpy)2(dcbpyH2), adsorbed on ZnO displays an even higher fraction of protonated oxygens, having almost a 1:1 intensity relation between the peaks at higher and lower binding energy.14 The Ru(bpy)2(dcbpyH2) dye therefore appears to be binding in a mode where all COOH groups are protonated. Why is the intensity relation different for the Ru(dcbpyH2)2(NCS)2 dye than for the Ru(bpy)2(dcbpyH2) dye? The 1:2.2 intensity relation between the protonated oxygens and the signal at the position of bridging oxygens indicates a mixture between a bonding mode where the carboxyl group is protonated and a bridge bonding mode. This is in line with theoretical studies of formic acid adsorbed onto a Zn (10-10) surface, suggesting that different bonding modes may be possible and that, depending on surface coverage, there may be a mixture of them.28 The bonding geometry differences between the Ru(dcbpyH2)2(NCS)2 and the Ru(bpy)2(dcbpyH2) dyes could, e.g., be due to the difference in pH between the dye solutions. Another contributing factor is the possibility of formation of Zn2+/dye aggregates for the Ru(dcbpyH2)2(NCS)2 dye. For this dye, monolayer adsorption and dye aggregation are probably two competing processes occurring at the same time. There may therefore well be fractions of aggregates present in the adsorbed dye layer even for short sensitizing times. In such a case, the O1s spectrum would be a sum of the contributions from adsorbed molecules and aggregates. S2p and N1s. In Figure 5, S2p spectra from sensitized ZnO (senstitizing time 25 min) and TiO2 are shown. The spectrum obtained for Ru(dcbpyH2)2(NCS)2 on ZnO is seen to differ substantially from that obtained when using TiO2 as a substrate. In the case of Ru(dcbpyH2)2(NCS)2 on TiO2, the obtained spectrum is close to that expected having predominantly one type of S species (resolved into its 1/2 and 3/2 spin-orbit contributions).25 The spectrum obtained for Ru(dcbpyH2)2(NCS)2 on ZnO, on the other hand, contains at least two S components: one at almost the same binding energy as for Ru(dcbpyH2)2(NCS)2 on TiO2 and one shifted toward higher binding energy.
10106 J. Phys. Chem. B, Vol. 106, No. 39, 2002
Figure 6. N1s core level spectrum of (a) Ru(dcbpyH2)2(NCS)2 on ZnO (b) Ru(dcbpyH2)2(NCS)2 on TiO2 (hν ) 758 eV.) The binding energy was calibrated with respect to the bipyridine N1s signal (the peak at higher binding energy).
The shift toward higher binding energy may result from an interaction with surface Zn atoms, indicating that the molecule has a tilted geometry, exposing one thiocyanate group directly to the surface. The conclusion that an interaction between the dye molecule and the surface atoms exists is supported by measurements on samples with very low dye coverage. For samples sensitized with a much less dye coverage (down to minutes, 0.2 monolayers) a spectrum similar to that in Figure 5 (containing two S components) was observed. The higher binding energy S2p component in Figure 5 may, however, also contain contributions from S interacting with Zn2+ in Zn2+/dye aggregates. In fact, the shape of this peak did not change appreciably with longer sensitizing times. Differences in the thiocyanate electronic structure between Ru(dcbpyH2)2(NCS)2 on ZnO and on TiO2 are also seen in the N1s spectra (Figure 6). Two N1s peaks are expected: one at lower binding energy originating from the thiocyanate ligand, and one from the bipyridine ligands at higher binding energy (intensity ratio of 1:2). As seen in Figure 6, the N1s signal from the dye adsorbed on TiO2 consists of two clearly resolved peaks, while the N1s signal for the dye on ZnO consists of one bipyridine peak and a broad tail toward lower binding energy containing the NCS contribution. The intensity relation is 1:2 for both Ru(dcbpyH2)2(NCS)2 on ZnO and on TiO2. Valence Levels. As discussed above, the oxygen, sulfur, and nitrogen core spectra show that for a low dye coverage (equal to or less than a monolayer) the surface molecular structure of the Ru(dcbpyH2)2(NCS)2 molecule differs between the ZnO and TiO2 substrates. As we have previously noted,24 the frontier electronic structure of the metal complex has a large NCS content. Therefore, any influence of the electronic structure of this ligand is likely to directly affect the valence levels. The valence level spectra of Ru(dcbpyH2)2(NCS)2 adsorbed onto ZnO and TiO2 are shown in Figure 7 together with the valence level spectra for the plain, unsensitized oxides. The outermost peak of the dye (the highest occupied molecular orbital, HOMO, level) is clearly distinguished at 1.3 eV, and the energy position of this peak with respect to the valence band of the substrate is similar in both cases. This indicates that the energy matching between the dye (the HOMO) and oxide (the valence band edge) is similar for ZnO and TiO2. In addition, although the position of the HOMO level is similar in the two cases, small differences in the peak shapes may be detected (Figure 7, inset). The peak obtained for Ru(dcbpyH2)2(NCS)2 adsorbed onto ZnO is somewhat broader
Westermark et al.
Figure 7. Valence level spectra of Ru(dcbpyH2)2(NCS)2 on ZnO (upper) and (b) on TiO2 (lower) (solid lines). The dashed lines show the valence levels of the plain nanostructured films. In the inset, a closeup of the outermost region of the spectra of the dye-sensitized and plain nanostructured films is shown. (hν ) 150 eV, binding energy calibration with respect to the bipyridine N1s signal via the Zn3d and Ti3p signals.)
than that obtained for Ru(dcbpyH2)2(NCS)2 adsorbed onto TiO2. Ru-polypyridine complexes are often interpreted within an octahedral symmetry in which the HOMO contains three degenerate orbitals (the t2g set). However, for the Ru(dcbpyH2)2(NCS)2 molecule, the symmetry is lowered and the three orbitals are split in energy. Thus, in line with the differences observed in the N1s and S2p spectra, the broadening of the peak for Ru(dcbpyH2)2(NCS)2 adsorbed on ZnO may be rationalized in terms of this splitting, whose size is likely to be dependent on the ligand configuration. Here it is also interesting to note that despite the broadening of the valence levels discussed above, the Ru 3d5/2 core level peak does not change as seen by an identical fwhm ) 0.80 eV at both the ZnO and the TiO2 surfaces. Changes in the orbital structure of the HOMO levels are likely to influence the spectral properties of the dye. Indications of such effects are observed in the UV-Vis absorption and photoelectrochemical measurements.7,12 Conclusions Ru(dcbpyH2)2(NCS)2 adsorbed onto nanostructured ZnO has been investigated by photoelectron spectroscopy, PES. For this system, formation of Zn2+/dye aggregates has previously been shown to occur, and such formation was also observed in the present study. Different sensitizing times were compared in order to examine this process, as well as to find a preparation leading to lower dye coverage corresponding to the preparation for an efficient solar cell. The ZnO films having low, mono- or submonolayer amounts, surface coverage were compared with those of the most efficient dye-sensitized system to date, i.e., Ru(dcbpyH2)2(NCS)2 adsorbed onto nanostructured TiO2. Even for a low dye coverage of the ZnO surface, we find that there are differences in the surface molecular structure compared to the corresponding TiO2 system. The differences were deduced from the chemical shifts in the core levels, in particular for the O1s, N1s, and S2p signals, and are interpreted as differences in molecular adsorption geometry. They may also appear as a consequence of dye aggregation even for the low coverages. The electronic structure of the thiocyanate ligand is found to be affected by the adsorption to the ZnO surface. This is
Adsorption of Dyes on Nanostructured ZnO interpreted as an effect of an interaction between the thiocyanate sulfur and zinc sites on the surface or with dissolved zinc ions during dye sensitization. This interaction also has implications on the electronic structure of the dye valence levels, resulting in a broadening in the HOMO level structure. However, the energy matching between the dye and ZnO was found to be similar to that of the dye/TiO2 system. Acknowledgment. This work was supported by the Swedish Research Council (VR) and the Foundation for Strategic Research, Sweden (SSF). References and Notes (1) O’Regan, B.; Gra¨tzel, M. Nature 1991, 353, 737. (2) Hagfeldt, A.; Gra¨tzel, M. Chem. ReV. 1995, 95, 49. (3) Hagfeldt, A.; Gra¨tzel, M. Acc. Chem. ReV. 2000, 33, 269. (4) Nazeeruddin, M. K.; Kay, A.; Rodicio, I.; Humphry-Baker R.; Mu¨ller, E.; Liska, P.; Vlachopoulos, N.; Gra¨tzel, M. J. Am. Chem. Soc. 1993, 115, 6382. (5) Hagfeldt, A.; Didriksson, B.; Palmqvist, T.; Lindstro¨m, H.; So¨dergren, S.; Rensmo, H.; Lindquist, S.-E. Sol. Energy Mater. Sol. Cells 1994, 31, 481. (6) Matsumura, M.; Matsudaira, S.; Tsubomura, H.; Takata, M.; Yanagida, H. Ind. Eng. Chem. Prod. Res. DeV. 1980, 19, 415. (7) Redmond, G.; Fitzmaurice, D.; Gra¨tzel, M. Chem. Mater. 1994, 6, 686. (8) Be´dja, I.; Kamat, P. V.; Hua, X.; Lappin, A. G.; Hotchandani, S. Langmuir 1997, 13, 2398. (9) Rensmo, H.; Keis, K.; Lindstro¨m, H.; So¨dergren, S.; Solbrand, A.; Hagfeldt, A.; Lindquist, S.-E.; Wang, L. N.; Muhammed, M. J. Phys. Chem. B 1997, 101, 2598. (10) Keis, K.; Magnusson, E.; Lindstro¨m, H.; Lindquist, S.-E.; Hagfeldt, A. Sol. En. Mater. Sol. Cells. 2002, 73, 51. (11) Asbury, J. B.; Wang, Y. Q.; Lian, T. Q. J. Phys. Chem. B 1999, 103, 6643. (12) Keis, K.; Lindgren, J.; Lindquist, S.-E.; Hagfeldt, A. Langmuir 2000, 16, 4688.
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