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Spillover-Induced Chemisorption of Amino Acid on Silver Surfaces Xueying Zhao,† Hao Yan,‡ Xiuwen Tu,§ R. G. Zhao, and W. S. Yang* The Mesoscopic Physics Laboratory and Department of Physic, Peking University, Beijing 100871, China Received October 11, 2002. In Final Form: January 2, 2003
Introduction Interfaces of organic molecules with inorganic substrates exist widely in nanoscience and technology. For example, in molecular electronic devices,1,2 in chemical or biological sensors,3,4 and in organic-mediated nanocrystals and supercrystals,5 the organic molecular layers are bound to the inorganic electrodes or substrates. It has been shown recently that soft materials such as DNA may be used as templates in nanofabrication of devices6,7 and that in heterogeneous enantioselective catalysis the enantioselectivity of catalytically active surfaces is usually induced by adsorption of some chiral molecules,8 such as amino acids.9,10 Organic/inorganic interfaces also play crucial roles in biomaterials and biocompatible materials.11 Moreover, it has even been pointed out that organic molecules, such as proteins, can alter inorganic microstructures of ceramics and semiconductors, etc., thus offering a very powerful tool for the design of novel hybrid materials.12 In these applications, the organic molecules must be, obviously, either chemisorbed on the surface or immobilized via other molecules that are chemisorbed on the surface. However, this is not always the case. For instance, Au, on one hand, has been used widely in molecule-mediated self-assembly of nanostructures13 and amino acids, on the other hand, have big potential in this respect, as just mentioned, adsorption of many amino acids, such as glycine, on Au surfaces is physisorption in character.14 Could the adsorption behavior of a given
molecule/substrate system be changed? We show here that this is indeed possible: glycine can be chemisorbed on Ag areas of the nanostructured Ag/Cu surfaces, while on pure Ag surfaces it is only physisorbed. Although we have studied glycine adsorption on both Ag/Cu and Au/Cu surfaces, here only adsorption on the Ag/Cu surface is chosen, because it has been known that in the Ag/Cu surfaces Ag atoms do not intermix with Cu atoms but stay on top of the surface,15,16 and this fact makes the phenomena clearer and their interpretations easier. Experimental Details The experiment was carried out in a UHV-STM (ultrahigh vacuum scanning tunneling microscopy) system that has been reported and used in a series of recent investigations on adsorption of amino acids on Cu surfaces.17,18 Briefly, in the STM experiment the bias voltage is applied to the sample and the tip is grounded. The tip is made of pure W wires with a diameter of 0.5 mm with electrochemical etching. The constant current mode of the STM was used throughout the work, and the scanning rate was from 200 to 2000 Å/s. Images given here were acquired at room temperature with the ac mode (same as the ac input mode of oscilloscopes, sometimes also called differential or localcontrast enhanced mode). The Cu(001) samples were cleaned by cycles of argon ion sputtering followed by annealing at 670 K. The Ag deposition source was a Ta-foil oven filled with a piece of Ag, and during deposition the Cu samples were also kept at room temperature. For preparation of the Ag/Cu(001) surface, the Ag deposition rate was set to 0.1 monolayer/min, but in preparation of the Ag/mica sample, the deposition rate was about 2 orders of magnitude higher. The glycine deposition source was also a Ta-foil oven but filled with the commercial glycine powder, which had the purity better than 99.0% and was used without further purification. The source was degassed in situ for about 10 min at about 370 K before the first deposition. During deposition the source was kept at about 400 K and the Cu samples, facing the opening of the oven, were at room temperature. The maximum dose was 10 langmuirs (1 langmuir ) 1.33 × 10-4 Pa‚s), and the rate was slightly below 2 langmuirs/min. The sample and source temperature were monitored with a PtRhPt and a copper-constantan thermocouple, respectively.
Results and Discussion * To whom correspondence may be addressed. E-mail: wsyang@ pku.edu.cn. † Present address: Department of Chemistry, University of Houston, Houston, TX. ‡ Present address: Department of Physics, Kansas State University, Manhattan, KS. § Present address: Department of Physics and Astronomy, University of California, Irvine, CA. (1) Chen, J.; Reed, M. A.; Rawlett, A. M.; Tour, J. M. Science 1999, 286, 1550. (2) Collier, C. P.; Wong, E. W.; Belohradsky´, M.; Raymo, F. M.; Stoddart, J. F.; Kuekes, P. J.; Williams, R. S.; Heath, J. R. Science 1999, 285, 391. (3) Kong, J.; Franklin, N. R.; Zhou, C.; Chapline, M. G.; Peng, S.; Cho, K.; Dai, H. Science 2000, 287, 622. (4) Cui, Y.; Wei, Q.; Park, H.; Lieber, C. M. Science 2001, 293, 1289. (5) Murray, C. B.; Kagan, C. R.; Bawendi, M. G. Annu. Rev. Mater. Sci. 2000, 30, 545. (6) Braun, E.; Eichen, Y.; Sivan, U.; Ben-Yoseph, G. Nature 1998, 391, 775. (7) Quake, S. R.; Scherer, A. Science 2000, 290, 1536. (8) Lorenzo, M. O.; Baddeley, C. J.; Muryn, C.; Raval, R. Nature 2000, 404, 376. (9) Zhao, X.; Zhao, R. G.; Yang, W. S. Langmuir 2000, 16, 9812. (10) Orme, C. A.; Noy, A.; Wierzbicki, A.; McBride, M. T.; Grantham, M.; Teng, H. H.; Dove, P. M.; DeYoreo, J. J. Nature 2001, 414, 775. (11) Jones, F. H. Surf. Sci. Rep. 2001, 43, 75. (12) Stupp, S. I.; Braun, P. V. Science 1997, 277, 1242. (13) Ulman, A. Chem. Rev. 1996, 96, 1533. (14) Zhao, X.; Yan, H.; Zhao, R. G.; Yang, W. S. Langmuir 2002, 18, 3910.
As the background of the present investigation, we recall that on the basis of recent investigations a coherent picture of the behavior of glycine adsorbed on Cu surfaces has emerged, which includes the following: (i) Cu surfaces at room temperature can make glycine deprotonated into glycinate and chemisorbed on them and (ii) the anionic adsorbates can form three different phases, that is, the 2D gas, 2D solid, and chain phases, where the molecules are in the standing, flat-lying, and unidentate conformation, respectively.18 To see how glycine adsorbates on Ag surfaces behave, we have studied the adsorption of glycine on the Ag/Cu(001) and Ag/mica surfaces. Typical STM images of the Ag/Cu(001) surface are given in panels a-c of Figure 1. These images confirm the conclusions15 that at submonolayer coverage, Ag atoms segregate into islands of (15) Sprunger, P. T.; Lægsgaard, E.; Besenbacher, F. Phys. Rev. B 1996, 54, 8163. (16) McMahon, W. E.; Hirschorn, E. S.; Chiang, T.-C. Surf. Sci. 1992, 279, L231. (17) Zhao, X.; Wang, H.; Zhao, R. G.; Yang, W. S. Mater. Sci. Eng., C 2001, 16, 41; Zhao, X.; Wang, H.; Yan, H.; Gai, Z.; Zhao, R. G.; Yang, W. S. Chin. Phys. 2001, 10, S84 and references therein. (18) Zhao, X.; Yan, H.; Zhao, R. G.; Yang, W. S. Langmuir 2003, 19, 809 and references therein.
10.1021/la0266886 CCC: $25.00 © 2003 American Chemical Society Published on Web 05/17/2003
Notes
Figure 1. (a) Low-magnification STM image (500 Å × 500 Å, 1.0 V, 0.6 nA) of the Cu(001) surface with about 0.7 monolayers of Ag atoms deposited. The two dark lines on the left (marked with white arrows) are surface steps. Ag atoms substitute only the first layer Cu atoms and form the c(2 × 10)-Ag islands (the brighter areas) separated by slightly lower and thus darker regions, which consist of Cu atoms and a small amount of Ag atoms. (b) Atom-resolved STM image (100 Å × 100 Å, 20 mV, 3 nA) of the Cu(001)c(2 × 10)-Ag structure, acquired from a Ag area in (a). Two of the four equivalent 〈320〉 directions are marked with black arrows. (c) Atom-resolved STM image (100 Å × 100 Å, 20 mV, 3 nA) acquired from the Cu(001) surface with about 1.3 monolayers of Ag deposited. In such cases the surface is covered by the same c(2 × 10)-Ag structure along with some islands (the upper-left portion) consisting of two nearly (111) Ag layers on the top.15 (d) STM image (180 Å × 180 Å, 1.0 V, 0.6 nA) of the same Ag/Cu(001) surface as that imaged in (a) but after adsorption of glycine. Note that the adsorbed glycine molecules form chains lying in the 〈110〉 direction. (e) STM image (230 Å × 230 Å, -1.0 V, 0.6 nA) obtained after further deposition of glycine onto the surface imaged in (d). Note that in this case the molecular chains on the c(2 × 10)-Ag structure align in the 〈320〉 directions, whereas the Cu regions now coalesce into larger islands covered by the c(2 × 4)-glycine superstructure, rather than molecular chains (see the lower portion). (f) Same as (e), but from an area where chains in two equivalent 〈320〉 directions coexist.
the c(2 × 10) superstructure located within the Cu surface layer while the islands are separated by Cu regions with a small amount of alloyed Ag atoms, and that when the coverage is slightly higher than one monolayer, then in the areas covered by two Ag layers the Ag atoms form two (111) layers whereas the rest of the areas remain c(2 × 10). The STM images obtained from the Ag film deposited on mica look pretty much the same as the upper-left portion of Figure 1c and thereby are not shown here. We find that if the Cu(001) surface is entirely covered by Ag, such as
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Figure 2. (a, b) A pair of STM images (40 × 40 Å2, 7.0 nA) acquired simultaneously from the same surface as that in Figure 1e. The bias voltage was -100 mV for (a) and -10 mV for (b), and thus the tunneling resistance for (a) was much higher than that for (b). As a result, the molecular chains were imaged in (a), whereas essentially only the c(2 × 10)-Ag structure beneath the chains was imaged in (b). Note that the c(2 × 10)-Ag structure in (b) was covered by a layer of molecules and thus the imaging quality was worse, even with the lower tunneling resistance. To show what imaged in (b) is indeed the c(2 × 10)-Ag structure, given in (c) is the same image as in (b) but with a c(2 × 10) quasi-hexagonal lattice superimposed and seven Ag atoms marked. (d) Atom-resolved image of the clean c(2 × 10)-Ag structure (40 × 40 Å2), for comparison with (b), as well as (c). The small orientation difference with (c) was a result of thermal drift.
the one imaged in Figure 1c, never could any features of glycine adsorbates be seen on the surface at room temperature, no matter how much glycine was deposited toward it. Moreover, for the glycine/Ag/mica system the situation is exactly the same: no sign of glycine adsorbates could be found on the Ag/mica surface. Actually, this is not surprising because it has been known that glycine adsorbates are in the zwitterionic form and thus are physisorbed on Ag surfaces.19 However, if the Ag/Cu(001) surface is not entirely covered by Ag, such as the one imaged in Figure 1a, we found, surprisingly, that features of glycine adsorbates can cover the entire surface at room temperature. Specifically, at lower coverage the surface is uniformly covered by chains lying exclusively in the 〈110〉 directions of the substrate and having a width of about 5 Å (see Figure 1d), just like the molecular chains observed from the glycine/ Cu systems,18 whereas at the saturation coverage the narrow Cu areas on the surface prior to deposition now coalesce into larger patches covered by the typical Cu(001)c(2 × 4)-glycine superstructure,17 and the chains are now aligned in the four equivalent 〈320〉 directions (see panels e and f of Figure 1). Comparing the chains in these images with the molecular chains seen from the glycine/ Cu surfaces,18 one could hardly deny that all of these are the same type of molecular chains, which have been shown to consist of glycine molecules in the unidentate conformation connected by hydrogen bonds.18 Now, with the images in Figure 2, we show that the molecular chains in panels e and f of Figure 1 are adsorbed on the c(2 × 10)-Ag (19) Stewart, S.; Fredericks, P. M. Spectrochim. Acta, Part A 1999, 55, 1641.
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Figure 3. (a) Low-magnification STM image (470 Å × 470 Å, -1.0 V, 3.0 nA) of the clean Cu(111) surface with about 0.7 monolayers of Ag deposited, where the Ag atoms stay on top of the first Cu layer and form islands of the (8 1, -1 8)-Ag structure. In this scale the (8 1, -1 8)-Ag structure is honeycomblike, whereas uncovered Cu(001)1 × 1 is flat. (b) Atom-resolved STM image (100 Å × 100 Å, 20 mV, 20 nA) of the (8 1, -1 8)-Ag structure. (c) STM image (150 Å × 150 Å, 1.0 V, 0.6 nA) obtained from the same surface but after glycine deposition, showing that the (8 1, -1 8)-Ag area of the surface was also covered by chains of glycine molecules. Note that in this case both the chains and the honeycomb-like pattern are visible.
layer. The images in panels a and b of Figure 2 were obtained simultaneously from the same place but with different tunneling resistances: when the tunneling resistance was high, the molecular chains were imaged (see Figure 2a), while under the low tunneling resistance the STM tip almost completely ignored the chains and imaged essentially only the underneath c(2 × 10)-Ag structure (see Figure 2b). To believe that which is imaged in Figure 2b is indeed the c(2 × 10)-Ag structure, one may compare the images in panels c and d of Figure 2, although the quality of the former is a bit worse, as one would expect. In the case of Ag/Cu(111), our images also confirm the conclusion reported previously; that is, Ag atoms stay on the surface and segregate into islands of the (8 1, -1 8)Ag structure.16 Similarly, deposition of glycine onto a Ag/ Cu(111) surface not completely covered by Ag (see Figure 3a) can also make its (8 1, -1 8)-Ag areas (see Figure 3b) covered by molecular chains (see Figure 3c). The difference is that in the present case the corrugation and its “wavelength” are larger and longer, respectively, than those of the c(2 × 10)-Ag structure, and thus in Figure 3c one can see simultaneously the chains as well as the underneath (8 1, -1 8)-Ag structure. To see if the glycine molecules are chemisorbed on the Ag areas of these surfaces, we annealed them at different temperatures. Surprisingly, on both Ag/Cu(001) and (111) surfaces the chains persisted after being annealed at 450 K for more than 5 min. This indicates9,17 that the binding energy of the adsorbed molecules must be not lower than 1.4 eV per molecule (or 135 kJ/mol) and, in turn, that the adsorption of the glycine molecules on the Cu(001)c(2 × 10)-Ag and Cu(111)(8 1, -1 8)-Ag areas of the two surfaces must be chemisorption. Moreover, our observations show that the ability of the Ag areas on these nanostructured Ag/Cu surfaces in chemisorbing glycine persists at least until Cu occupies only 1/10 of the total surface area,
Notes
although, as mentioned above, pure Ag surfaces alone are unable to make glycine chemisorbed on them. To find out what was responsible for the changed adsorption behavior of the Ag patches on the nanostructured Ag/Cu surfaces, we have to consider those mechanisms that have already been known to be able to bring some intrinsic chemical property changes to the metal(s) involved in many nanostructured surfaces. As we are dealing with bimetallic surfaces, the first mechanism that has to be considered is the formation of the so-called heteronuclear metal-metal bonds, because formation of such bonds may induce significant changes in the band structure of the involved metals and, in turn, may result in surfaces with novel chemical and catalytic properties.20 For instance, it is possible to use Au to modify the reactivity of Ni with respect to the steam re-forming process.21 Second, we have to consider if the changed adsorption behavior is related to the small thickness of the Ag patches on these nanostructured Ag/Cu surfaces, because it has been known that in the case of Au clusters supported by titania the changed catalytic property of Au is related to a quantum size effect with respect to the thickness of the clusters.22 Third, as it has been known for very long that preoxidized Ag and Au surfaces can make formic acid deprotonated and the resulting formate can be chemisorbed on the Ag and Au surfaces23 and it has been shown very recently that bonding of glycinate on the Cu surface is very similar to that of formate,24 we also have to consider if the Ag layers were preoxidized, albeit we did not do that intentionally. Of course, in this context one may still point out or even find out more possible mechanisms that have to be considered. However, as mentioned above, the c(2 × 10)-Ag surface that does not have Cu patches on it (see Figure 1c) is unable to make glycine chemisorb on it. Using this as the control experiment, we must conclude that any mechanism, including the three mentioned above, that would cause a change in the intrinsic chemical property of the relevant metals should be ruled out as a possible mechanism responsible for the changed adsorption behavior of the Ag patches on the nanostructured Ag/Cu surfaces or, in other words, the glycinate molecules chemisorbed on the Ag patches could not be native but must be transported or spilt-over from the Cu patches on the surface. To understand why spillover happens in the present cases, we recall that in the field of heterogeneous catalysis nanostructured surfaces such as those in the present investigation are involved everywhere and spillover, not accidentally, frequently plays important roles.25,26 Spillover, by definition, is such a process that involves the transport of active species sorbed or formed on a first surface onto another surface that does not, under the same condition, sorb or form the active species.25,26 According to this definition we argue that chemisorption of glycine on the Ag areas on the heterogeneous Ag/Cu surfaces must be a result of spillover. Our arguments are the following: (i) As experiments23 and calculations27 concluded that (20) Rodrigues, J. A. Surf. Sci. Rep. 1996, 24, 223. (21) Besenbacher, F.; Chorkendorff, I.; Clausen, B. S.; Hammer, B.; Molenbroek, A. M.; Nørskov, J. K.; Stensgaard, I. Science 1998, 279, 1913. (22) Valden, M. V.; Lai, X.; Goodman, D. W. Science 1998, 281, 1647. (23) Sim, W. S.; Gardner, P.; King, D. A. J. Phys. Chem. 1996, 100, 12509 and references therein. (24) Hasselstro¨m, J.; Karis, O.; Nyberg, M.; Pettersson, L. G. M.; Weinelt, M.; Wassdahl, N.; Nilsson, A. J. Phys. Chem. B 2000, 104, 11480. (25) Pajonk, G. M. In Handbook of Heterogeneous Catalysis; Ertl, G., Kno¨zinger, H., Weitkamp, J., Eds.; VCH: Weinheim, 1997; Vol. 3, pp 1064-1077. (26) Conner, W. C.; Falconer, J. L. Chem. Rev. 1995, 95, 759.
Notes
adsorption of formate on Ag surfaces is chemisorption and, on the other hand, calculations24 concluded that the bonding orbital of glycinate on metal surfaces are strongly localized and thus are similar to those of the former,24 Ag surfaces should be able to chemisorb glycinate. (ii) However, surface-enhanced Raman spectroscopy experiment found that glycine molecules adsorbed on pure Ag surfaces are in the zwitterionic form,19 thus indicating that Ag surfaces are unable to make glycine deprotonate into glycinate. (iii) Calculations, on one hand, predicted that only glycinate can be chemisorbed on Cu surfaces whereas neutral and zwitterionic glycine can only be physisorbed,28 and experiments, on the other hand, found that adsorption of glycine on Cu surfaces is indeed chemisorption,17 indicating that Cu surfaces are able to make glycine deprotonate into glycinate. (iv) As for the driving force of the spillover, it has been known that at low coverage the glycinate molecules tend to take the “standing” conformation on the Cu surface,18 making their dipole moments parallel to each other and thus the intermolecular interactions repulsive. As a result of this, spillover can reduce the electrostatic energy of the system. Moreover, spillover can make the adsorbates distributing more randomly on the entire surface or, in other words, can increase the entropy and thus reduce the free energy of the system. Of course, we have also noticed that calculations showed that the bonding of formate on Cu(110) is stronger than that on Ag(110),27 and this may be true also for the bonding of glycinate on Cu and Ag surfaces, providing a negative driving force to the spillover. Obviously, spillover depends on the result of the competition of these factors, but our results indicate that it is favored in the present cases. Therefore, we conclude that although pure Ag surfaces have the ability to chemisorb glycinate molecules, they are unable to make glycine molecules deprotonate into glycinate molecules by themselves, and thereby in order to make use of this ability they have to rely on the spillover source, that (27) Casarin, M.; Maccato, C.; Vittadini, A. J. Chem. Soc., Faraday Trans. 1998, 94, 797. (28) Wilson, W. D.; Bisson, C. L.; Schaldach, C. M. J. Colloid Interface Sci. 1997, 187, 201.
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is, the nearby Cu nanopatches, to provide them with glycinate molecules. Finally, it should be emphasized that although spillover is a well-studied phenomenon but only in the field of heterogeneous catalysis.25,26 Moreover, with the potential applications in mind, we would reiterate the following two facts: First, the required surface area of the spillover source is very small; in the present case, only 0.1 monolayers of Cu or less are required to be on the surface. Second and more importantly, taking advantage of spillover-induced chemisorption does not require change or sacrifice of any intrinsic chemical property of the surface. These two characters make spillover-induced chemisorption a potentially important means in applications, especially in the fields such as biocompatible materials, biosensors, organic-mediated nanometer scale fabrications or growth of nanocrystals, and heterogeneous enantioselective catalysis, or even in development of proteinmediated materials, where chemisorption of organic molecules on inorganic surfaces is of crucial importance.1-12 Summary We show that although glycine can only be physisorbed on pure Ag surfaces, it can be made to chemisorb on Ag areas of both the nanostructured Ag/Cu(001) and (111) surfaces, provided a small portion of the surface remains Cu. Using the same Ag/Cu(001) and (111) surfaces, but without surface Cu atoms as the control surfaces and considering the closely related calculations and experimental studies published recently, it is concluded that spillover of deprotonated glycine molecules from Cu to Ag is responsible for this new adsorption behavior of glycine on Ag. Such spillover-induced chemisorption is expected to have wide technical applications in many fields. Acknowledgment. Helpful discussions with Professor Youchang Xie in the Department of Chemistry, Peking University is gratefully acknowledged. This work was supported by the Ministry of Science and Technology of China (under #2001CB610500) and the National Natural Science Foundation of China (Grant #10134030). LA0266886
