J. Phys. Chem. C 2007, 111, 10523-10527
10523
Photodecomposition of Pentacene Films on Atomically Controlled SrTiO3(001) Surfaces Yuji Matsumoto,* Hideomi Koinuma, and Takeo Ohsawa Materials and Structures Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan ReceiVed: March 26, 2007; In Final Form: April 30, 2007
Photodecomposition of pentacene films grown on the atomically controlled SrTiO3(001) (STO) surfaces was investigated by atomic force microscopy. The photodecomposition was found to occur only when the coverage of the pentacene film was less than 1 ML on the TiO2-terminated STO(001). When the oxygen adsorption is a rate-limiting step, the decomposition rate is nearly proportional to the surface area uncovered with pentacene island, while at the last stage of the decomposition it is in proportion not to the coverage of the pentacene islands but to the perimeter of the pentacene islands. The SrO-terminated STO(001) surface exhibits very little activity in the photodecomposition of the pentacene. The exponential decay of the rate constant with the SrO coverage suggests that it is attributed to some chemical effect such as the formation of Sr hydroxide at the SrO-terminated surface.
1. Introduction Since the discovery of the photoelectrolysis of water, using a titanium dioxide (TiO2) crystal,1 TiO2 has attracted keen attention in view of solar-to-chemical and electricity conversion,1,2 and photochemical environmental cleaning.3 For the better performance of such photochemical properties, new families of niobate,4 tantalate,5 and vanadate6-based photocatalysts have being developed recently besides titanate-based ones. Some of them contain more metal elements, and consequently their crystal structure becomes more complex. For example, SrTiO3 (STO), which is one of the most simple complex oxide photocatalyst, can be regarded as a natural superlattice of SrO and TiO2 atomic layers along the direction. This allows us to distinguish two possible terminations of the SrO and TiO 2 at the topmost layer of STO(001). The former is a kind of insulator, while the latter is part of the anatase crystal lattice. Hence, an interesting question arises whether photochemical properties depends not only on crystal orientations, impurities, and defects, but also on the choice of the termination layer, SrO or TiO2, on the STO(001) surface. This could be a general question for the complex oxide photocatalysts. In fact, the SrOterminated STO(001) surface was found to exhibit less photodecomposition of pentacene than the TiO2-terminated one.7 To be able to investigate photochemical properties depending on such atomic-scale controlled structure and composition in photocatalysts, we have studied laser molecular beam epitaxy (MBE) of single crystalline oxide thin films such as epitaxial anatase,8 rutile TiO2,9 and related Ti oxide films.10 The key to the high quality of such oxide films is the use of several singlecrystal substrates with atomically flat terraces and steps such as STO,11 sapphire,12 and rutile.13 The ideal layer-by-layer growth of films is attained on these single-crystal substrates with the aid of reflection high-energy electron diffraction (RHEED) intensity monitoring. On the basis of these technological advantages, we have investigated photodecomposition of organic pentacene films * To whom correspondence should be addressed.
[email protected]. Fax: +81-45-924-5377.
E-mail:
grown on the atomically controlled STO(001) surfaces on which a c-axis-oriented crystalline film of pentacene is known to grow. In the present research, a new evaluation technique is employed because catalytic activity for single-crystal surfaces is inherently difficult to evaluate, owing to their small surface area; the photodecomposition of pentacene film is directly observed by atomic force microscopy (AFM) and is quantitatively analyzed. In our previous works, it was found that the photodecomposition of pentacene films occurs on the STO(001) in air or O2 atmosphere but not in vacuum.14 In the present study, we focus on the reaction kinetics for the photodecomposition of pentacene films on well-defined single-crystal STO(001) surfaces not only to understand the photodecomposition process but also to quantitatively discuss the termination effect on it. 2. Experimental Section Selective etching of SrO layer by buffered HF solution treatment gives us atomically flat STO(001) completely terminated with TiO2 layer. The SrO-terminated STO(001) was prepared by the laser MBE method. The deposition of the SrO layer was precisely controlled (0 < θSrO < 1) by the observation of RHEED intensity oscillation,15 synchronized with moving a physical mask over the STO surface during the deposition, as will be described later. The substrate temperature, oxygen partial pressure, and KrF excimer laser conditions were 400 °C, 1 × 10-5 Torr, and 0.5 J/cm2 (1 Hz), respectively. The films were then annealed at 400 °C in atmospheric O2 to compensate for the lack of oxygen in the films. Both the SrO- and TiO2terminated STO(001) surface exhibited a sharp RHEED pattern and atomically smooth surface as was confirmed by RHEED and AFM observations. Pentacene was deposited at room temperature by K-cell evaporation (the cell temperature was 230 °C). The X-ray diffraction (XRD) pattern for the thicker films showed the growth of c-axis oriented pentacene crystalline in thin film phase. The coverage of the pentacene film was controlled by the evaporation time to be about 1 ML.
10.1021/jp072365c CCC: $37.00 © 2007 American Chemical Society Published on Web 06/20/2007
10524 J. Phys. Chem. C, Vol. 111, No. 28, 2007
Matsumoto et al.
Figure 2. Area of pentacene islands, S, is plotted as a function of the illumination time, t, when the initial coverage is 1.5, 1, and 0.3 ML, respectively.
Figure 1. A typical series of AFM images (10 µm × 10 µm) for the photodecomposition of the 1 ML pentacene film on the TiO2-terminated STO(001): UV illumination time t ) 0 (a), t ) 60 min (b), t ) 75 min (c), and t ) 105 min (d), respectively.
The UV lamp (λ ) 254 nm and 0.6 mW/cm2) was illuminated in air for a while, and the remaining coverage of pentacene, S, was then quantitatively evaluated by AFM. This procedure was repeated and the value of S was plotted as a function of the illumination time, t. The scan area of all the AFM images, S0, is 100 µm2 (10 µm × 10 µm). 3. Results and Discussion 3.1. Photodecomposition of Pentacene on TiO2-Terminated STO(001). Figure 1 is a typical series of AFM images (10 µm × 10 µm) for the photodecomposition of the 1 ML pentacene film on the TiO2-terminated STO(001): UV illumination time t ) 0 (a), t ) 60 min (b), t ) 75 min (c) and t ) 105 min (d), respectively. As shown in Figure 1a, the pentacene film was confirmed to grow in the layer-by-layer fashion in this thickness regime of less than 1 ML and the second layers of pentacene partially grown on the first layers were observed in this specimen. The height of islands was unique to be 1.5-1.6 nm, as is reported in the previous literatures16 and the quantitative analysis is thus possible just by evaluating the area of pentacene islands.17 With illumination time, the area of islands gradually decreased to indicate that the pentacene was photochemically decomposed. It should be noticed that the second layers still survived even when the total coverage of pentacene was reduced during the photodecomposition. This fact suggests that the penacene islands are not restructured according to the total coverage during the photodecomposition, owing to its little diffusion ability at RT. In Figure 2, the remaining area of pentacene, S, is plotted as a function of the illumination time, t, when the initial coverage of the pentacene is 1.5, 1, and 0.3 ML, respectively. In cases of the coverage less than 1 ML, the photodecomposition of pentacene films proceeded with t. The reaction kinetics could be characterized by three reaction regimes: an induction of the reaction at the initial stage of the UV illumination, followed by an abrupt acceleration of the reaction and a slow-down of the reaction at the last stage when the coverage of pentacene less than 0.2 ML. In our previous work,14 low-energy electron diffraction spot intensity from the
STO surface was found to linearly increase with the surface area uncovered with pentacene island during the photodecomposition. This fact indicates that there remain no pentacene derivatives that are partially oxidized on the STO surface. The subsequent decomposition of such derivatives or their desorption from the surface is thus so fast that the first oxidation of pentacene molecules seems to be a rate-limiting step in the present case. Interestingly, in the case of the initial coverage of the pentacene above 1 ML no photodecomposition occurred in the present experimental condition. This result coincides with the experience as is often pointed out in the application that the photodecomposition ability deteriorates when the surface is too contaminated. Reaction Kinetics When the CoVerage of the Pentacene Is High. In our previous work,14 it was found that there are two kinds of pentacene on the STO: one is forming the crystal island, as observed by AFM, and the other is the precursor state adsorbed on the STO surface. The latter is photochemically more reactive than the former. At the initial stage of the photodecomposition, both species, that is, pentacene molecules not only from the crystal island but also from the adsorbed state, are therefore decomposed simultaneously so that the apparent decomposition rate focusing on the change in the pentacene islands should be lower as a kind of a induction period. After the induction period, the reaction may be accelerated because the precursor state of pentacene is no longer adsorbing on the STO surface. Furthermore, this interpretation is very plausible for the reaction behavior when the initial coverage of the pentacene is as low as 0.6 ML as will be later shown in Figure 6, where the induction period was still clearly observed owing to the presence of precursor state of pentacene at the initial stage of the decomposition. On the basis of these results and because the reaction kinetics should be more complex at the induction period, we focus here on the reaction kinetics after the induction period. It is assumed that when the coverage of the pentacene islands is relatively high the oxygen adsorption is a rate-limiting step and the decomposition rate, υ, should be proportional to the surface area uncovered with pentacene island, S0 - S, as expressed by eq 1 as follows
υ)-
dS ) k1‚(S0 - S) dt
ln(S0 - S) ) k1t + C
(1) (2)
Photodecomposition of Pentacene Films
J. Phys. Chem. C, Vol. 111, No. 28, 2007 10525
Figure 3. Dependence of the ln(S0 - S) on the illumination time, t. Data points to be well fitted by the linear function, the slope of which (i.e., the rate constant) is deduced to be 3.23 ( 0.32[h-1].
Figure 4. The rate constant plotted as a function of the illumination time, t: b for the rate eq 3 and 0 for the rate eq 4, respectively.
where k1 is a rate constant and C is a integration constant. In this case, ln(S0 - S) must be proportional to the reaction time, t, according to eq 2, and in fact a linear relationship between them was confirmed as shown in Figure 3. The rate constant is deduced to be 3.23 ( 0.32[h-1] for t ) 0.5 to 1.25 h. Reaction Kinetics for the Last Stage of the Photodecomposition. As already mentioned above, the pentacene islands seem not to be restructured, and any holes and ditches are not observed in the island during the photodecomposition. One possible reaction model to explain these observations is that some active chemical species such as O2- and/or O produced on the STO surface as well as a hole itself may be consumed preferentially near the edge of pentacene islands. If this is the case, the decomposition rate should be in proportion to the perimeter of the pentacene islands, L, as expressed by eq 3. Hashimoto et al. reported on a similar AFM observation of photodecomposition of monolayer stearic acid prepared on a rutile (110) single crystal by the Langmuir-Blodgett method.18 In this case, however, the islands of the stearic acid were randomly decomposed not only from their edges but also from the inside of the islands, giving a kind of the first-order reaction kinetics as expressed by eq 4
υ)-
dS ) k2‚L dt
(3)
υ)-
dS ) k3‚S dt
(4)
Here, k2 and k3 are the rate constant for each reaction scheme postulated at present. To clarify which reaction model was more plausible, we examined the reaction kinetics for the last stage of the photodecomposition in which the O2 adsorption would not be a rate-limiting step any longer. In Figure 4, the rate
Figure 5. Preparation of the SrO-terminated STO with a linear modulation of the SrO coverage (0 < θSrO < 1) on one sample. (a) RHEED intensity oscillation; (b) schematic illustration of experimental procedure employed.
Figure 6. The area of pentacene islands was plotted as a function of illumination time for the samples with different SrO coverage, respectively.
constants of k2 ) υ/L and k3 ) υ/S are plotted as a function of t. It is the value of k2 that is almost constant with time of 1.5 to 2.0 h, but not for k3, which tells us that the reaction model as expressed by eq 3 would be more probable for the present photodecompositon of pentacene. Note that these reaction kinetics, however, cannot be applied to the regime of the acceleration of the reaction. In situ AFM observation of photodecomposition process, which will be our later work, could reveal more clearly the reaction mechanism. 3.2. Termination Effect on the Photodecomposition. For the SrO-terminated STO(001) surface, SrO was deposited on the TiO2-terminated STO(001) by laser MBE. The coverage of SrO, θSrO, was estimated by monitoring the RHEED intensity oscillation as shown in Figure 5a in which one period of the intensity oscillation corresponds to the 1 ML growth of SrO layer.12 To precisely control the SrO deposition, the physical mask was linearly moved with deposition time over the sample
10526 J. Phys. Chem. C, Vol. 111, No. 28, 2007
Matsumoto et al.
Figure 7. The rate constant is plotted as a function of the SrO coverage. It is found that the rate constant is exponentially reduced with the SrO coverage.
surface, giving a linear modulation of the SrO coverage (0 < θSrO < 1) on one sample as illustrated by Figure 5b.19 The advantage of this sample preparation is that the UV lamp can be illuminated simultaneously on STO(001) surfaces with different SrO coverage to ensure exactly the same experimental conditions for the photodecompostion of pentacene. In Figure 6, the area of pentacene islands was plotted as a function of illumination time for the samples with different SrO coverage, respectively. There could be found a very clear tendency that the photodecomposition of pentacene is gradually more suppressed when the SrO coverage becomes higher. Surprisingly, the covering with just the monolayer of SrO can make the STO(001) surface almost inactive in the photodecomposition of pentacene. At first, it should be pointed out that the effect of processing surface damage in the laser MBE is a less possible reason for the suppression of the photodecomposition at the present case. According to our experiment of the photodecomposition on laser MBE-grown anatase films, the anatase film exhibited enough photodecomposition ability comparable to that for the STO single-crystal surface. Thus, the SrO effect observed here should be intrinsic. The tendency could be quantitatively analyzed in terms of the rate constant estimated in the accelerated reaction regime, irrespective of the initial coverage of the pentacene. In Figure 7, the rate constant is plotted as a function of the SrO coverage. It is found that the rate constant is exponentially reduced with the SrO coverage. It should be pointed out that the rate constant even for θSrO ) 0 is 1.09 ( 0.10[h-1], which is significantly smaller than that of 3.23 ( 0.32[h-1] for the fresh TiO2terminated STO(001). Ohnishi et al. reported that a sizable amount of SrO segregates onto the surface by annealing the TiO2-terminated STO(001) at temperatures as low as 300 °C20. In the present sample preparation, the sample experienced the annealing process at 400 °C so that a very small but not negligible amount of SrO (∼10%) segregated onto the STO surface even without depositing SrO. The exponential decay of the rate constant with the SrO coverage suggests that such a very small amount of SrO should be fatal to the photodecomposition process, resulting in the reduction of the rate constant even for the nominal coverage of SrO, θSrO ) 0, in this sample. One possible reason for such inefficient photodecomposition at the SrO-terminated STO is that the monolayer of SrO acts as an insulating layer to physically hinder the process of electron transfer from the bulk to adsorbed O2 molecules as shown in Figure 8a. It is because the consumption of photoexcited electrons is indispensable for the steady-state photooxidation to maintain the charge balance in the photocatalyst. In fact, a
Figure 8. Schematic illustrations of the termination effect on the photodecomposition of pentacene on the STO(001) surface. (a) Physically blocking effect model; (b) chemical effect model of Sr hydroxide formation.
similar effect of the monolayer SrO on the electron transfer at the interface between STO and LaAlO3(LAO) was reported by Nishimura et al.21 In the photooxidation process, the photoexcited holes can travel from the bulk onto the surface via the 2p valence band of O, irrespective of the terminations. In contrast, the electron travels via the 3d conduction band of Ti and is trapped at the Ti ion site. For the TiO2 termination, the trapped electron could reach the topmost layer and have a strong interaction with the O2 molecule but not for the SrO termination. However, this simple physically blocking effect should give the rate constant linearly reduced with SrO coverage and thus cannot explain the observed exponential decay of the rate constant. Another possible reason is that the SrO surface itself or its related compound, such as, the Sr hydroxide complex on the surface, prevents the photodecomposion as shown in Figure 8b. In fact, the preferential formation of the Sr hydroxide complex was reported by Iwahori et al.22 When an amount of water is exposed to the clean STO(001), the topmost surface, which is composed of both SrO and TiO2 layers in vacuum, the Sr3d peak in X-ray photoelectron spectroscopy changes but not the Ti2p peak. As a result, a strong friction contrast is observed between SrO- and TiO2-terminated surfaces by friction force microscope. They attributed such a strong friction contrast to the preferential reaction of water with the SrO layer. The hydroxide complex might be less active for the photodecomposition of pentacene islands. The exponential decay of the rate constant with the SrO coverage suggests that the Sr hydroxide complex acts as a recombination center for electron-hole pairs. Further in situ study at an ultra-high vacuum condition to avoid the formation of the Sr hydroxide complex will be needed to reveal the origin of the termination effect. 4. Conclusion In this study, the reaction kinetics for the photodecomposition of pentacene films and its termination effect on atomically designed single-crystal STO(001) surfaces were investigated by AFM. It was quantitatively demonstrated that an atomic-scale
Photodecomposition of Pentacene Films surface condition is seriously influential on the photodecomposition ability on the STO. The initial coverage of pentacene film is very critical and the 1 ML coverage is enough to completely suppress the photodecomposition. The STO surface should be terminated with the TiO2 layer for the photodecomposition. The TiO2-terminated surface is about 20 times more active than the SrO-terminated one. Acknowledgment. This work has been partially supported by CREST of JST (Japan Science and Technology Agency). Supporting Information Available: XRD pattern and Fourier transform IR spectrum for a thicker pentacene film grown on STO(001), RHEED patterns and AFM images for the STO(001) surfaces with different SrO coverage, together with a schematic illustration of the sample prepared in this study, and the reference data of the photodecomposition on the anatase film are included. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Fujishima, A.; Honda, K. Nature 1972, 238, 37-38. (2) (a) O’Regan, B.; Gra¨tzel, M. Nature 1991, 353, 737-740. (b) Gra¨tzel, M. Nature 2001, 414, 338-344. (3) (a) Linsebigler, A.; Lu, G.; Yates, J. T., Jr. Chem. ReV. 1995, 95, 735-758. (b) Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W. Chem. ReV. 1995, 95, 69-96. (4) Domen, K.; Yoshimura, J.; Sekine, T.; Tanaka, A.; Onishi, T. Catal. Lett. 1990, 4, 339-343. (5) Kato, H.; Asakura, K.; Kudo, A. J. Am. Chem. Soc. 2003, 125, 3082-3089.
J. Phys. Chem. C, Vol. 111, No. 28, 2007 10527 (6) Yu, J.; Kudo, A. AdV. Funct. Mater. 2006, 16, 2163-2169. (7) Matsumoto, Y.; Ohsawa, T.; Takahashi, R.; Koinuma, H. Thin Solid Films 2005, 486, 11-14. (8) Murakami, M.; Matsumoto, Y.; Nakajima, K.; Makino, T.; Segawa, Y.; Chikyow, T.; Ahmet, P.; Kawasaki, M.; Koinuma, H. Appl. Phys. Lett. 2001, 78, 2664-2666. (9) Yamamoto, Y.; Matsumoto, Y.; Koinuma, H. Appl. Surf. Sci. 2004, 238, 189-192. (10) Ohsawa, T.; Nakajima, K.; Matsumoto, Y.; Koinuma, H. Appl. Surf. Sci. 2006, 252, 2603-2607. (11) Kawasaki, M.; Takahashi, K.; Maeda, T.; Tsuchiya, R.; Shinohara, M.; Ishiyama, O.; Yonezawa, T.; Yoshimoto, M.; Koinuma, H. Science 1944, 266, 1540-1542. (12) Yoshimoto, M.; Maeda, T.; Ohnishi, T.; Koinuma, H.; Ishiyama, O.; Shinohara, M.; Kubo, M.; Miura, R.; Miyamoto, A. Appl. Phys. Lett. 1995, 67, 2615-2617. (13) Yamamoto, Y.; Nakajima, K.; Ohsawa, T.; Matsumoto, Y.; Koinuma, H. Jpn. J. Appl. Phys. 2005, 40, L511-514. (14) Ohsawa, T.; Matsumoto, Y.; Koinuma, H. Jpn. J. Appl. Phys. 2005, 44, 4142-4144. (15) Takahashi, R.; Matsumoto, Y.; Ohsawa, T.; Lippmaa, M.; Kawasaki, M.; Koinuma, H. J. Cryst. Growth 2002, 234, 505-508. (16) Bouchoms, I. P. M.; Schoonveld, W. A.; Vrijmoeth, J.; Klapwijk, T. M. Synth. Met. 1999, 104, 175-178. (17) Matsumoto, Y.; Nakajima, K.; Ohsawa, T.; Koinuma, H. Meas. Sci. Technol. 2005, 16, 199-202. (18) Hashimoto, K.; Irie, H.; Fujishima, A. Jpn. J. Appl. Phys. 2005, 44, 8269-8285. (19) Takahashi, R.; Kubota, H.; Murakami, M.; Yamamoto, Y.; Matsumoto, Y.; Koinuma, H. J. Comb. Chem. 2004, 6, 50-53. (20) Ohnishi, T.; Shibuya, K.; Lippmaa, M.; Kobayashi, D.; Kumigashira, H.; Oshima, M.; Koinuma, H. Appl. Phys. Lett. 2004, 85, 272-274. (21) Iwahori, K.; Watanabe, S.; Kawai, M.; Kobayashi, K.; Yamada, H.; Matsushige, K. J. Appl. Phys. 2003, 93, 3223-3227. (22) Nishimura, J.; Ohtomo, A.; Ohkubo, A.; Murakami, Y.; Kawasaki, M. Jpn. J. Appl. Phys. 2004, 43, L1032-1034.
