Adsorption of Co-Phthalocyanine on the Rutile TiO2(110) Surface: A

Article pubs.acs.org/JPCC

Adsorption of Co-Phthalocyanine on the Rutile TiO2(110) Surface: A Scanning Tunneling Microscopy/Spectroscopy Study Nobuyuki Ishida*,† and Daisuke Fujita†,‡ †

Global Research Center for Environment and Energy Based on Nanomaterials Science (GREEN), National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan ‡ Advanced Nanocharacterization Unit, National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan ABSTRACT: We investigated the adsorption properties of Co-phthalocyanines (CoPc's) on the rutile TiO2(110) surface using scanning tunneling microscopy and spectroscopy. Analytical results showed that the adsorption properties strongly depend on the deposition conditions. Under specific deposition conditions (elevated substrate temperatures and low deposition rates), the molecules are immobilized on the surface without aggregation up to the full monolayer coverage. The substrate temperature plays a key role in determining the bonding nature between the molecules and the substrate. The immobilized molecules introduce gap states at the interface and locally decrease the surface band gap to an energy value that is lower than that of the absorption band of the CoPc thin film. Coverage dependence studies show that the adsorption structures are dominated by molecule−substrate interactions for the first monolayer growth and by a complex interaction between molecules in the first and second monolayers for the second monolayer growth.

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INTRODUCTION Molecular adsorption on TiO2 surfaces has attracted much attention due to its wide range of applicability, including use in solar cells1,2 and for photocatalysis.3−6 Among such applications, dye-sensitized solar cells (DSSCs) have been rigorously studied for their potential as a lower-cost alternative to conventional photovoltaic cells.1,2,7,8 In DSSCs, the adsorption properties of dye molecules, including bonding nature and molecular distribution, as well as the relevant electronic properties at the interface, play a fundamental role in determining carrier transport and injection processes9−11 and thereby device performance. Understanding these properties and designing an adsorption configuration at the molecular level are essential for further improvement of DSSCs. Until now, a lot of work has been performed on wellcharacterized single-crystal TiO2 surfaces with adsorbed organic molecules for the atomistic-level understanding of molecular adsorption.12−15 However, the molecular species studied are rather limited (e.g., carboxylic acids and alcohols), while studies on dye molecules are still lacking. Among dye molecules, phthalocyanine (Pc) and its derivatives are robust and have good light-harvesting properties, and thus they are expected to be an alternative to the ruthenium polypyridyl dyes for DSSC applications.16,17 In this work, we used scanning tunneling microscopy (STM) and spectroscopy (STS) to characterize the adsorption properties of Co-phthalocyanines (CoPc's) on the rutile TiO2(110) surface. In particular, we investigated the dependence of the molecular adsorption on the substrate temperature during deposition, the deposition rate, and the coverage. Here, © 2012 American Chemical Society

we show that the growth and organization of the molecules are strongly dependent on the deposition conditions.18,19 The key for determining the adsorption structures is to evaluate the extent of the contribution of various interactions, such as molecule−substrate and intermolecular interactions.

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EXPERIMENTAL METHODS

We performed the experiments in an ultra-high-vacuum (UHV) system. The UHV system is primarily composed of a preparation chamber and a main chamber with base pressures of 1.5 × 10−8 and 7.0 × 10−9 Pa, respectively. The preparation chamber is equipped with two electron bombardment heating systems for the samples and the tips, respectively,20 an evaporator for molecular deposition, and an ion gun. The main chamber is equipped with an STM head (Omicron VTSPM). STM/STS measurements were performed at room temperature (RT) using Pt/Ir tips. Sample bias voltages of 1.5− 2.0 V and tunneling currents of 0.05−0.3 nA were used as tunneling parameters. We acquired tunneling spectra using a voltage modulation of 30 mVrms and a lock-in amplifier to obtain differential conductance. The rutile TiO2(110) singlecrystal surfaces (Shinkosha) were cleaned by repeated cycles of Ar+-ion sputtering (1.2 keV, 5 min) and annealing at about 1100 K for 10 min. The CoPc's (Sigma-Aldrich, 97% purity) were deposited from a stainless steel crucible after sufficient Received: April 3, 2012 Revised: August 10, 2012 Published: August 30, 2012 20300

dx.doi.org/10.1021/jp303152c | J. Phys. Chem. C 2012, 116, 20300−20305

The Journal of Physical Chemistry C

Article

the diffusion of molecules on the surface, suggesting that the molecules weakly interact with the substrate, for example, only through van der Waals interaction: that is, the molecules are physisorbed on the surface. In contrast, deposition with an elevated substrate temperature (400 K) resulted in immobilization of the molecules on the surface, as shown in Figure 2b. This indicates that the interaction between the molecules and substrate is strengthened due to the thermal energy. We also investigated the adsorption behaviors under higher substrate temperature conditions (up to ∼500 K), and similar results were obtained. In addition, we explored another deposition condition, that is, deposition of the molecules at RT, followed by postannealing treatment at 400 K for 20 min. The results are similar to the case of the deposition with an elevated substrate temperature (400 K). However, postannealing treatment at higher temperature resulted in the decrease in the coverage of the immobilized molecules, suggesting that some molecules were desorbed from the surface during the postannealing process. Figure 3 shows the dependence of adsorption of CoPc's on the deposition rate. The substrate temperature during the

degassing. The deposition rate was measured with a quartz crystal microbalance.

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RESULTS AND DISCUSSION On the (1 × 1) phase of rutile TiO2(110), the O rows protruding over the surface, so-called bridge-bonded O (Obb), extend along the [001] direction and the 5-fold coordinated Ti rows run in between the Obb rows (Figure 1a). During a

Figure 3. Dependence of the adsorption of CoPc's on the deposition rate. Deposition was performed at a rate of (a) 0.5 nm/h for 10 min (scale bar = 20 nm) and (b) 0.05 nm/h for 45 min. The substrate temperature was kept at 400 K for both cases.

Figure 1. Ball models of (a) the rutile TiO2(110) surface and (b) a Co-phthalocyanine molecule.

vacuum annealing process, Obb vacancies are introduced, as shown in Figure 1a. The CoPc molecule consists of four isoindole units linked by N atoms and a Co atom at the center (Figure 1b). Figure 2 shows the dependence of the adsorption of the CoPc's on the substrate temperature. The deposition rate and time were fixed at 0.05 nm/h and 20 min, respectively. Figure 2a shows an STM image in which the molecules were deposited while keeping the substrate at RT. A lot of spikes resulted from

deposition was fixed at 400 K since the molecules are immobilized on the surface under this condition. Figure 3a shows an STM image where the molecules were deposited at a rate of 0.5 nm/h for 10 min. A majority of the molecules were immobilized and could be imaged, which is in contrast to the case of RT deposition (see Figure 2a). However, many spikes, the signature of mobile molecules, were observed. In addition, the molecules aggregated and formed clusters at some locations (some of them are indicated by circles in the figure). A deposition rate one tenth of the previous one (0.05 nm/h for 90 min) resulted in a significant reduction of the spikes due to the disappearance of the mobile molecules. In addition, the molecules randomly adsorbed without aggregation. This deposition rate dependence indicates that some rate-limited process is involved in the formation of immobilized adsorption of the molecules. The substrate temperature and the deposition rate dependence studies show that the molecules are immobilized on the surfaces without aggregation under specific deposition conditions (elevated substrate temperatures and low deposition rates). The immobilization implies that the interaction between the molecule and the substrate increases. Since the molecules are already mobile upon the RT deposition (see Figure 2a), the increase in the diffusion rate due to the thermal energy and

Figure 2. Dependence of the adsorption of CoPc's on the substrate temperature during deposition. Molecular deposition was performed while (a) keeping the substrate at RT (scale bar = 20 nm) and (b) heating the substrate at 400 K. 20301



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by assuming the position of maximum height corresponding to the center of the molecules, that is, the position of Co atoms, we cannot specify the direction of the four ligands of the molecules (position around the center atoms) from just this STM image. This is due to the lack of clear 4-fold symmetry, which is expected from the molecular shape. For this reason, we were not able to construct a structural model of the adsorbed molecules. The shapes of the molecules at the type-A and typeB positions are slightly different from each other. On careful inspection, we found that some molecules exhibit a similar shape and thus are expected to have the same adsorption structure; however, we were not able to classify all the molecules into clear-cut groups. The nearly equal probability of having molecules at the type-A and type-B positions suggests the similar energetic stability of the molecules at these positions. The observation of molecular hopping between these positions during the scanning, like those indicated by arrows in Figure 4b, is also consistent with this finding. Usually, Pc's and metal Pc's (MPc's) exhibit a protruding four-lobed structure in STM images that reflects the molecular structure of the four ligands (a crosslike shape).21−23 In our images, however, the immobilized molecules exhibit a squarelike shape without the protruding lobes. Previous studies have shown that the shape of MPc's is drastically changed from crosslike to squarelike when removing eight H atoms of the ligands by applying voltage pulses from an STM tip.24,25 This is due to the formation of chemical bonds of the molecular orbital with the substrate via dehydrogenation. Since the images of the dehydrogenated MPc's are similar to those of the immobilized molecules, we expect that similar chemical bonds via dehydrogenation could be the cause of the immobilization. In that case, the planar shape of the molecules is changed to a curved structure with its center shifted upward and the end C atoms form chemical bonds with either 5-fold coordinated Ti or Obb. According to this model, each molecule releases eight H atoms. However, we did not observe a noticeable increase in bridging OH groups. Considering that the molecules are immobilized under high-temperature conditions, the H atoms might be desorbed from the surface as hydrogen molecules. To further support this hypothesis, additional study using X-ray photoelectron spectroscopy measurements and description by first-principles calculation is required. To evaluate the electronic properties of the immobilized molecules, we performed STS measurements on surfaces, like the one shown in Figure 4. Figure 5 shows the typical differential tunneling conductance spectra obtained at a pristine TiO2 and at an immobilized molecule site. Note that tunneling conductance is roughly proportional to the local density of states at the surfaces.26 In the empty states, both spectra display an increase of the signal. This can be attributed to the empty states of the conduction band for the spectrum at the TiO2 site, and either those or the lowest unoccupied molecular orbital for the one at the molecule site. The onset of the component at the small positive voltage indicates that the location of the Fermi level (0 V in the spectrum) is close to the conduction band edge, which is consistent with the n-type character of vacuumannealed TiO2.12 In the filled states, the dI/dV signal at the TiO2 site remains small, whereas at the molecule site, a significant increase was observed for lower voltages of less than −1.0 V. The nonzero conductance value at the TiO2 site within the band-gap region could be due to the current flowing through the surface states 27 and/or a dopant-induced component, as was observed on the n-type GaAs(110)

resultant hopping to the energetically favorable adsorption sites cannot explain the phenomenon. Some conformational change of the molecules through the formation of chemical bonds with the substrate can be expected. We will discuss a possible mechanism involved in the formation of the chemical bonds in the following. Figure 4a shows a high-resolution STM image of the immobilized molecules on the TiO2(110) surface. The bright

Figure 4. (a) High-resolution STM image of immobilized CoPc's on the TiO2(110) surface (scale bar = 5 nm). (b) Duplicated STM image of (a) with solid lines and circles and triangles that mark the position of Ti rows and maximum height position of the molecules, respectively.

and dark rows along the [001] direction of the surface arise from the 5-fold coordinated Ti and Obb, respectively.12 The bright spots on the dark rows correspond to the Obb vacancies. The density of the Obb vacancies is about 0.07 ML, where 1 ML is defined as the density of the 1 × 1 units, 5.2 × 1018 m−2. Since many Obb vacancies are seen at the pristine TiO2(110) sites without molecular adsorption, these vacancies are considered to play a small role in the immobilization of the molecules. The CoPc's appear as bright protrusions, displaying a lateral size of around 1.7 nm. Considering the lateral size, the molecules are expected to adsorb flat on the surface or tilted slightly. Figure 4b is the duplicated image of Figure 4a with solid lines and circles and triangles that mark the position of the Ti rows and the maximum height position of the molecules, respectively. Twenty molecules display their maximum height position on the Ti rows (marked with triangles, type A), whereas 21 molecules appear on the O rows (marked with circles, type B). Although the adsorption site can be determined 20302



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Figure 5. Differential tunneling conductance spectra obtained at the clean TiO2 site and at an immobilized molecule site.

surfaces.28,29 The significant increase of the signal at the molecule site indicates the formation of gap states leading to the local decrease of the surface band gap. We were not able to measure the extent of the broadening of the gap states in energy to the negative voltage side due to the instability of the tip under high negative voltages (less than −2.5 V). The spectra obtained on the molecules at type-A and type-B positions did not show a noticeable difference. The surface band gap at the molecule site can roughly be estimated as 1.2 eV, assuming that the nonzero conductance value at the molecule site in the voltage range from −1.0 to −0.4 V has the same origin as those at the TiO2 site. The estimated value is lower than the energy value of the absorption band of the CoPc thin film (around 680 nm, corresponding to 1.8 eV30). We also investigated the evolution of the molecular adsorption as a function of coverage, as shown in Figure 6. We used the deposition condition under which the molecules are immobilized on the surface (substrate temperature, 400 K; deposition rate, 0.05 nm/h). The coverage was controlled by adjusting the deposition time (from 5 to 310 min). In Figure 6a−d, we can see that, during the growth of the first monolayer, the molecules randomly adsorbed on the surface and the coverage increased in a nearly linear dependence with the deposition time. No preferential adsorption at the step edges was observed. Even when the coverage was close to the full monolayer and the intermolecular distances reached values less than 2.0 nm, we did not observe any ordering of the molecules such as those reported on metal surfaces.21−23 This indicates that intermolecular interaction does not play much of a role in determining the adsorption structure of the first monolayer under this deposition condition. Instead, the adsorption structures are dominated by the molecule−substrate interaction. After the completion of the first monolayer, the speed of the increase of the coverage with the deposition time drastically slowed down, most likely due to the decrease of the sticking coefficient. At the beginning of the second monolayer growth, bright spotswhich we tentatively attributed to the molecules of the second monolayerappeared randomly distributed on the surface, as shown in Figure 6e. On further deposition, twodimensional (2D) islands were formed at some locations, indicated by the arrows in Figure 6f. These islands became larger with further deposition, as shown in Figure 6g, and the molecules formed multiple wavy rows within the islands, as shown in Figure 6h, which is a magnified image of the area

Figure 6. Evolution of molecular adsorption as a function of coverage. (a−g) Images show a 100 × 100 nm2 of the surface after deposition for (a) 5, (b) 20, (c) 45, (d) 90, (e) 190, (f) 250, and (g) 310 min (scale bar = 20 nm). In (h), a magnified image of the region indicated by the square in (g) is shown (scale bar = 10 nm).

surrounded by a square in Figure 6g. The distance between the rows is about 1.6 nm. In addition, periodic modulation with about a 1 nm distance is evident along the row, although it is difficult to distinguish it in this image. Since the periodicity of 1 nm along the row is much smaller than the lateral size of the molecules, the molecules are likely to be tilted from the parallel position to the surface. This is consistent with the strong tendency of Pc's to form π-stacked aggregates.31 We did not see any correlation between the row direction and specific crystallographic directions of the TiO2(110). Therefore, the ordering in the adsorption configuration comes from a substantial contribution from the intermolecular interaction. The wavy character can be understood by considering 20303



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nonuniform adsorption structures (no ordering) in the first monolayer, indicating that the adsorption structure in the second monolayer is determined by a complex interaction between the molecules in the first and second monolayers. In our experiments, not all the surface was covered by the ordered structures: spikes arising from mobile molecules were observed at other areas. This suggests that nucleation sites (see Figure 6f) are needed for the formation of the ordered structures. We will discuss possible nucleation sites below. On clean rutile TiO2(110) surfaces after the vacuum annealing procedure, we observed raised corrugations extended over several Ti and O rows,32 as shown in Figure 7a (some of

Figure 8. STM images of CoPc/TiO2(110) surfaces where the molecules are deposited on a substrate kept at 330 K up to a full monolayer coverage (scale bar = 10 nm). Scans (a) and (b) were successively obtained at the same location.

the molecules were deposited with a substrate temperature of 330 K. The majority of the molecules could not be resolved individually, but an ordered structure was locally formed at the location indicated by a white arrow. Figure 8b shows the STM image successively obtained at the same location of Figure 8a (the scanning area is slightly shifted and distorted due to the drift). The ordered structure observed in Figure 8a almost disappeared (indicated by a white arrow), and similar ordered structures were newly formed at different locations (indicated by black arrows) where no ordering of the molecules was observed before. These observations suggest that mobile molecules can be ordered on the rutile TiO2(110) surfaces and these ordered structures are somehow stabilized by very weak interactions. Since there are always bright protrusions at the corner of the ordered structures, which can be attributed to immobilized molecules (some of them are indicated by circles in Figure 8b), the immobilized molecules are likely to play a key role in the stabilization. These findings might also support our hypothesis that the areas of raised corrugations on the clean TiO2 surfaces (see Figure 7a) could be the nucleation sites of the 2D ordered structure of the molecules in the second monolayer, as observed in Figure 6g, due to the following reason. On the region of raised corrugations, the molecules are mobile at the full monolayer coverage, as in Figure 7c, and thus might have formed the ordered structures like the one in Figure 8. The ordering of the molecules in the first monolayer, in turn, may have arranged the molecules in the second monolayer into the ordered structures.

Figure 7. (a) Typical STM image of a clean rutile TiO2(110) surface (scale bar = 5 nm). In addition to Ti and O rows, bright features that broaden over several nanometers are observed. (b, c) Molecular adsorption properties on the bright features (scale bar = 8 nm).

them are indicated by circles in the figure). Similar bright or dark features are commonly observed on semiconductor surfaces and can be attributed to the local fluctuation of the surface potential due to the existence of charged impurities in the subsurface regions.33,34 We consider Ti interstitials that are introduced into the crystal during the annealing procedure35,36 to be the most probable candidate for the impurity because the features did not disappear even after many cycles of sputtering and annealing. Interestingly, on some bright features (but not all), the adsorption of the CoPc's was hindered even when the coverage was close to the full monolayer, as shown in Figure 7b. Further deposition resulted in the filling of empty areas by mobile (only physisorbed) molecules, as shown in Figure 7c. Considering that the size of the bright features is comparable to the initial size of the 2D islands shown in Figure 6f, those features might be the nucleation sites of the 2D ordered structures observed in the second monolayer growth (see Figure 6g). Further observation may support this hypothesis, as we will explain below. Upon deposition with substrate temperatures less than 400 K, molecules are not immobilized but rather diffuse on the surface. Since mobile molecules disturb stable STM imaging, the coverage dependence has not been studied under such conditions. Nevertheless, we found that stable imaging was possible when the coverage was a full monolayer where the diffusion length is limited. Figure 8a shows an example where

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CONCLUSION We studied the adsorption properties of CoPc's on rutile TiO2(110) surfaces by means of STM/STS. We particularly focused on the dependence of the properties on the substrate temperature during deposition, the deposition rate, and the coverage. We showed that the adsorption properties strongly depended on the deposition conditions: deposition at RT resulted in mobile molecules, whereas at elevated temperatures, the molecules were immobilized on the surface. The immobilization could be due to the formation of chemical bonds with the substrate. At a low deposition rate, all the molecules were immobilized without aggregation up to the full monolayer coverage, whereas a relatively higher rate led to the coexistence of mobile and immobile molecules and molecular aggregates. The immobilized molecules locally introduced gap states and significantly reduced the surface band gap to an energy value that is lower than that of the absorption band of 20304



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CoPc thin film. The studies of the coverage dependence show that the adsorption structures are dominated by the molecule− substrate interaction for the first monolayer growth, and by a complex interaction between molecules of the first and second layers for the second monolayer growth. Our experimental results demonstrate that controlling the balance of such interactions is the key to designing an adsorption configuration as well as the relevant electronic properties at the interface.

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(26) Tersoff, J.; Hamann, D. R. Phys. Rev. Lett. 1983, 50, 1998. (27) Wendt, S.; Sprunger, P. T.; Lira, E.; Madsen, G. K. H.; Li, Z.; Hansen, J. O.; Matthiesen, J.; Blekinge-Rasmussen, A.; Laegsgaard, E.; Hammer, B.; Besenbacher, F. Science 2008, 320, 1755. (28) Feenstra, R. M.; Stroscio, J. A. J. Vac. Sci. Technol., B 1987, 5, 923. (29) Ishida, N.; Sueoka, K.; Feenstra, R. M. Phys. Rev. B 2009, 80, 075320. (30) Atta, N. F.; Amin, H. M. A.; Khalil, M. W.; Galal, A. Int. J. Electrochem. Sci. 2011, 6, 3316. (31) de la Torre, G.; Claessens, C. G.; Torres, T. Chem. Commun. 2007, 20, 2000. (32) Batzill, M.; Katsiev, K.; Gaspar, D. J.; Diebold, U. Phys. Rev. B 2002, 66, 235401. (33) Brown, G. W.; Grube, H.; Hawley, M. E. Phys. Rev. B 2004, 70, 121301. (34) Ebert, P. Surf. Sci. Rep. 1999, 33, 121. (35) Henderson, M. A. Surf. Sci. 1999, 419, 174. (36) Park, K. T.; Pan, M.; Meunier, V.; Plummer, E. W. Phys. Rev. B 2007, 75, 245415.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +81 29 859 2000, ext. 3874. Fax: +81 29 859 2801. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Carmen Pérez León for advice and revision of the manuscript. This work was supported by the MEXT Program for the “Development of Environmental Technology using Nanotechnology” and a Grant-in-Aid for Young Scientists B (23710137) from the Ministry of Education, Culture, Sports, Science and Technology of Japan.

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Adsorption of Co-Phthalocyanine on the Rutile TiO2(110) Surface: A

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