- Email: [email protected]
An STM study on the growth process of vapor-deposited hydroquinone adlayers on Rh(1 1 1) and Pt(1 1 1)
Chemical Physics Letters 399 (2004) 373–377 www.elsevier.com/locate/cplett
An STM study on the growth process of vapor-deposited hydroquinone adlayers on Rh(1 1 1) and Pt(1 1 1) Junji Inukai
a,b
, Mitsuru Wakisaka b, Kingo Itaya
b,c,*
a
b
NICHe, Tohoku University, 6-6-10 Aoba, Sendai 980-8579, Japan Department of Applied Chemistry, Faculty of Engineering, Tohoku University, 6-6-04 Aoba, Sendai 980-8579, Japan c CREST, JST, 4-1-8 Kawaguchi, Saitama 332-0012, Japan Received 10 August 2004; in final form 1 October 2004
Abstract Hydroquinone molecules were allowed to adsorb on Rh(1 1 1) and Pt(1 1 1) by vapor deposition. The strong interaction between p p two adjacent hydroxyl groups of neighboring molecules led to the formation of commensurate ( 7 · 7)R19.1 and incommensup p rate ( 2.56 · 2.56)R16 adlayer phases on Rh(1 1 1) and Pt(1 1 1), respectively. The formation processes of HQ adlayers on Rh(1 1 1) and Pt(1 1 1) were very different from each other, possibly due to the difference in lattice parameters of the two metal surfaces. 2004 Elsevier B.V. All rights reserved.
1. Introduction The adsorption of organic molecules on metal surfaces is a major subject of modern interfacial chemistry [1–3]. We previously investigated the adsorption of aromatic molecules such as benzene, naphthalene, anthracene, and coronene on well-defined Rh(1 1 1) [4–7], Pt(1 1 1) [4–6], Cu(1 1 1) [6,8], and Au(1 1 1)[9,10] surfaces by using scanning tunneling microscopy (STM) [6]. Recently, we showed that both intermolecular and molecule–substrate interactions play important roles in the formation of ordered adlayers of benzene derivatives. We investigated the adlayers of methyl-substituted benzene derivatives, such as toluene, o-xylene, p-xylene, and mesitylene, formed on Rh(1 1 1) in HF solution [11,12]. For p-xylene, we also investigate its adlayers on Rh(1 1 1) and Pt(1 1 1) formed by vapor deposition [13,14]. Although the atomic distances on Rh(1 1 1) and Pt(1 1 1) surfaces are different, namely 0.269 and *
Corresponding author. Fax: +81 22 214 5380. E-mail address: [email protected] (K. Itaya).
0009-2614/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2004.10.023
0.278 p nm, respectively, the same adlayer phase of c(2 3 · 4)rect was observed on both Rh(1 1 1) and Pt(1 1 1) in solution and vacuum [14], indicating that the intermolecular interaction between p-xylene molecules is small. Hydroquinone (HQ) is a well-studied molecule, which undergoes electrochemical oxidation/reduction reactions in solution [15]. Hubbard and co-workers investigated the adlayers of HQ on Pt electrodes by employing thin-layer electrochemical cells [3,16,17], low energy electron diffraction (LEED) [3,18,19], Auger electron spectroscopy [3,18,19], and high-resolution electron energy loss spectroscopy (HREELS) [3,18–20]. Wilson et al. [21] performed STM studies of HQ by using a UHV compatible cell, wherein a drop of ultrapure water containing HQ was evaporated from a Pt(1 1 1) surface in an Ar atmosphere. Soto et al. [22] investigated the adlayers of HQ on Pd(1 1 1) and Pd(1 0 0) electrode surfaces by tandem electrochemistry, HREELS, and in situ STM. Those reports showed that adsorbed HQ molecules formed monolayers with an orientation parallel to the Pt and Pd single-crystal surfaces,
374
J. Inukai et al. / Chemical Physics Letters 399 (2004) 373–377
and that the hydrogen in hydroxyl group was dissociated from the molecule. Very recently, we reported the surface structures of the adlayers of HQ on p Rh(1 p 1 1) [23] and Pt(1 1 1) [24] to be commensurate ( 7 · 7)R19.1 and incommensurate (2.56 · 2.56)R16, respectively, regardless of whether they were formed in solution or in vacuum. Surprisingly, although the surface symmetries were different as stated above, the adlayer structures of HQ on both metal surfaces were identical [23,24]. Figs. 1a and b show structure models for the adlayers of HQ on Rh(1 1 1) and Pt(1 1 1), respectively [23,24]. In these models, the hydrogen in hydroxyl group is depicted to be dissociated from the molecule as reported previously [3,18–22]. The HQ molecules are adsorbed horizontally with respect to the Rh(1 1 1) and Pt(1 1 1) planes. On
Rh(1 1 1), HQ molecules are located at threefold sites so that all oxygen atoms are allowed to be adsorbed at threefold sites [25]. The strong interaction between two adjacent hydroxyl groups of neighboring HQ molecules with sp3 oxygen orbitals leads to the formation of the zigzag configuration of molecular binding (Fig. 1) [23,24]. The of HQ on Rh(1 1 1) and Pt(1 1 1) with p coverages p ( 7 · 7)R19.1 and (2.56 · 2.56)R16 phases were calculated to be 0.143 and 0.153, respectively, but the packing density of HQ on Rh(1 1 1) and Pt(1 1 1) coincided to be 0.379 nmol cm2 on both surfaces because of the identical adlayer structures [23,24]. In this study, we carried out an STM investigation of the formation process of HQ adlayers on Rh(1 1 1) and Pt(1 1 1) by vapor deposition. Perturbations were observed in the bonding between HQ molecules on Rh(1 1 1) and Pt(1 1 1).
2. Experimental Single-crystal beads of Rh and Pt, 3 mm in diameter, were made by crystallization of molten balls formed at the end of Rh and Pt wires in a hydrogen–oxygen flame [6]. The laser beam deflection method was used to determine the orientation of the single-crystal beads to expose the (1 1 1) planes, which were then mechanically polished with successively finer-grade diamond pastes down to 0.05 lm with an accuracy in the angle of <0.1. The vapor deposition of HQ (reagent grade, Kanto Chemical Co.) on Rh(1 1 1) and Pt(1 1 1) was carried out in a vacuum chamber, and the specimens produced were subsequently investigated by LEED and STM [23,24].
3. Results and discussion
Fig. 1. Structural models of ordered HQ adlayers on Rh(1 1 1) (a) and p p Pt(1 1 1) (b). Commensurate ( 7 · 7)R19.1 and incommensurate (2.56 · 2.56)R16 adlayer structures are shown in (a) and (b).
In our previous work, we continuously exposed Rh(1 1 1) to p-xylene in a vacuum chamber during the STM observation to follow time-dependent processes during the growth of ordered phases [13]. During the continuous dosing, adsorbed p-xylene molecules were very mobile [13]. The formation of the disordered adlayer was found at the first stage of adsorption of p-xylene [14]. As described below, the results obtained with HQ were in distinct contrast to those obtained with pxylene. Fig. 2 shows STM images of HQ molecules on Rh(1 1 1) after the vapor deposition. As can be seen in Fig. 2a, HQ molecules interact with each other even at a low coverage of approximately 0.05, in contrast to the case of p-xylene p [13,14]: linear chains of HQ molecules run in the 7 direction. The width of each chain is seen to correspond to 1–3 molecules, showing the existence of lateral interaction between the molecules.
J. Inukai et al. / Chemical Physics Letters 399 (2004) 373–377
375
Fig. 3 shows the STM images revealing the growth process of the HQ adlayer on Pt(1 1 1). At the first stage of adsorption with a coverage of 0.02 (Fig. 3a), the molecules are clearly seen to interact with each other as was observed on Rh(1 1 1) (Fig. 2). However, the growth process was different from that on Rh(1 1 1). In the case on Rh(1 1 1), p straight chains of molecules were seen to run in the 7 direction (Fig. 2a), whereas on Pt(1 1 1), the molecules form winding chains (Fig. 3a) [21]. On Rh(1 1 1), the molecular chains do not cross each other
Fig. 2. STM images of vapor-deposited HQ molecules on Rh(1 1 1). Solid arrows indicate close-packed directions of the Rh(1 1 1) subp strate. The dashed arrows indicate the 7 direction. (a) After exposure 7 to HQ at 5 · 10 Torr for 1 s (0.5 L). Scan area = 25 · 25 nm2. Tip bias = 0.50 V. Tunneling current = 1.0 nA. (b) After exposure at 5 · 107 Torr for 5 s (2.5 L). Scan area = 40 · 40 nm2. Tip bias = 0.35 V. Tunneling current = 1.3 nA.
The length of each chain is 10–20 nm. The molecular chains are all straight, exhibiting no curvatures. Fig. 2b shows an STM image of the monolayer of HQ on Rh(1 1 1). HQ molecules have been proposed to be adsorbed at threefold sites as depicted in Fig. 1a [23]. The observation with LEED clearly showed a p p ( 7 · 7)R19.1 pattern on the surface [23]. The ordered domains are narrow, 10–20 nm in length, apparently reflecting the growth process of the HQ layer on Rh(1 1 1) (Fig. 2a). At domain boundaries, some HQ molecules look brighter, probably because of the adsorption of those molecules on the monolayer of HQ. Interestingly, the domains of HQ on Rh(1 1 1) (Fig. 2b) are similar in shape to those of terephthalic acid on Pt(1 1 1) [26], which formed hydrogen bonding between the molecules.
Fig. 3. STM images of vapor-deposited HQ molecules on Pt(1 1 1). Solid arrows indicate close-packed directions of the Pt(1 1 1) substrate. (a) After exposure to HQ at 5 · 107 Torr for 2 s (1 L). Scan area = 25 · 25 nm2. Tip bias = 1.50 V. Tunneling current = 0.5 nA. (b) After exposure at 5 · 107 Torr for 6 s (3 L). Scan area = 20 · 20 nm2. Tip bias = 0.05 V. Tunneling current = 4.0 nA. (c) After exposure at 5 · 107 Torr for 12 s (6 L). Scan area = 25 · 25 nm2. Tip bias = 0.35 V. Tunneling current = 2.0 nA.
376
J. Inukai et al. / Chemical Physics Letters 399 (2004) 373–377
(Fig. 2a), while on Pt(1 1 1), some crossings of molecular chains are observed (Fig. 3a). As the coverage by HQ molecules on Pt(1 1 1) increased, the molecular chains began to align themselves in the direction rotated by approximately 16 with respect to the Pt atomic rows (Figs. 1b and 3b). When the full coverage was reached, the formation of a (2.56 · 2.56)R16 phase was confirmed by LEED [24]. Some HQ molecules, adsorbed on the monolayer, also look brighter on Pt(1 1 1). As can be seen in Fig. 3c, the domains of HQ were very small on Pt(1 1 1), namely 5 nm. On Rh(1 1 1), domain boundaries were very clearly developed (Fig. 2b), whereas on Pt(1 1 1), it is difficult even to distinguish domain boundaries, because HQ molecules at boundaries are interconnected (Fig. 3c). Even at the first stage of adsorption, HQ molecules were observed to interact each other both on Rh(1 1 1) and Pt(1 1 1), forming chains, which was not observed for p-xylene molecules: each p-xylene molecules were isolated at low coverages, showing little, if any, attractive interaction [14]. By using HREELS, Hubbard and co-workers [3,18–20] reported that on a Pt(1 1 1) electrode, the hydrogen atom in a hydroxyl group was dissociated from the molecule, because the OH band was absent. If this holds, the dissociated hydrogen atoms on Rh(1 1 1) and Pt(1 1 1) might be adsorbed between the two oxygen atoms in the neighboring HQ molecules. This would form a hydrogen bond, O H O, between the neighboring HQ molecules. By LEED, Hubbard and coworkers [3,18,19] reported a (3 · 3) phase for the HQ adlayer on Pt(1 1 1) with a packing density of only 0.276 nmol cm2, whereas we formed a (2.56 · 2.56)R16 phase with a packing density of 0.379 nmol cm2. The two structures are very different, thus, in our experiments, there exists a possibility that some hydroxyl groups might remain at the surface species on Rh(1 1 1) and Pt(1 1 1) to form hydrogen bonds. Wilson et al. [21] suggested that the formation of HQ chains may result from hydrogen bonding of the phenolic H in solution prior to dissociation. Our study showed that the identical chains are formed not only in solution but also in vacuum, which cannot be explained by the mechanism proposed by Wilson et al. The difference between the HQ adlayers on Rh(1 1 1) and Pt(1 1 1) can be geometrically explained in terms of the lattice parameters, 0.269 nm on Rh(1 1 1) and 0.278 nm on Pt(1 1 1). The lattice parameter of HQ adlayer must closely match with that of Rh(1 1 1) in order for the HQ molecules to be naturally situated at commensup rate sites: the HQ chains and domains grow in the 7 direction (Fig. 2). On the other hand, the lattice parameter on Pt(1 1 1) is slightly larger for the HQ adlayer, and therefore, HQ molecules cannot be situated at commensurate sites. In the process of forming a bond for a new HQ molecule on Pt(1 1 1), it will search for an incommensurate, but energetically more favorable, sur-
Fig. 4. High-resolution STM image of HQ monolayer on Pt(1 1 1) after exposure to HQ at 5 · 107 Torr for 12 s (6 L). Scan area = 5 · 5 nm2. Tip bias = 0.35 V. Tunneling current = 2.0 nA. Molecular structures of HQ are superimposed.
face site. Consequently, winding molecular chains are expected to grow (Fig. 3a). When the HQ coverage increases on Pt(1 1 1), the lateral interaction between adjacent HQ molecules causes the formation of the ordered, incommensurate adlayer structure (Figs. 3b and c). The formation process of the HQ adlayers shows that the interaction between molecules and Rh(1 1 1) and Pt(1 1 1) substrates are not very strong, but strong enough to produce differences on the two substrates. Fig. 4 shows a high-resolution STM image of a HQ adlayer on Pt(1 1 1) with HQ molecular structures superimposed. Molecules of HQ, each of which has a phenyl ring at the center and two hydroxyl groups, one on each side, are seen to form a bond between neighboring hydroxyl groups. The chains of HQ are seen to be bent as described above, which was never observed on Rh(1 1 1) (Fig. 2). Interestingly, not only two but even three hydroxyl groups are seen to interconnect with each other. In the case of HQ on Rh(1 1 1), the interconnection with three hydroxyl groups could not be formed because the molecular chains were never bent (Fig. 2). On the other hand, chains are bent on Pt(1 1 1), and the bending point with an sp3 oxygen orbital at the apex could serve as the interconnection with three molecules. The bonding between HQ molecules was found to be perturbed on different surfaces with different lattice parameters, due to the difference in organizing process of the adlayers.
4. Conclusions Molecules of HQ were allowed to adsorb on Rh(1 1 1) and Pt(1 1 1) by vapor deposition. The HQ
J. Inukai et al. / Chemical Physics Letters 399 (2004) 373–377
molecules were found to form commensurate p p ( 7 · 7)R19.1 and incommensurate (2.56 · 2.56) R16 adlayer structures on Rh(1 1 1) and Pt(1 1 1), respectively. The formation processes of HQ adlayers on the two surfaces were different from each other probably due to the difference in substrate lattice parameters. Acknowledgements This work was supported partially by the COE project, ÔGiant molecules and Complex Systems, 2004,Õ from the Ministry of Education, Culture, Sports, Science and Technology, Japan, and by Core Research for Evolutional Science and Technology organized by the Japan Science and Technology Corporation. The authors thank Dr. Y. Okinaka for his help in writing this manuscript.
References [1] J. Lipkowski, P.N. Ross (Eds.), Adsorption of Molecules at Metal Electrodes, VCH, New York, 1992. [2] A. Wieckowski (Ed.), Interfacial Electrochemistry, Marcel Dekker, New York, 1999. [3] A.T. Hubbard, Chem. Rev. 88 (1988) 633. [4] S.-L. Yau, Y.-G. Kim, K. Itaya, J. Am. Chem. Soc. 118 (1996) 7795. [5] S.-L. Yau, Y.-G. Kim, K. Itaya, J. Phys. Chem. B 101 (1997) 3547. [6] K. Itaya, Prog. Surf. Sci. 58 (1998) 121.
377
[7] J. Inukai, M. Wakisaka, K. Itaya, Jpn. J. Appl. Phys. 43 (2004) 4554. [8] L.-J. Wan, K. Itaya, Langmuir 13 (1997) 7173. [9] S. Yoshimoto, R. Narita, K. Itaya, Chem. Lett. (2002) 356. [10] S. Yoshimoto, R. Narita, M. Wakisaka, K. Itaya, J. Electroanal. Chem. 532 (2002) 331. [11] K. Suto, M. Wakisaka, M. Yamagishi, L.-J. Wan, J. Inukai, K. Itaya, Langmuir 16 (2000) 9368. [12] K. Suto, J. Inukai, K. Itaya, J. Electroanal. Chem. 550–551 (2003) 105. [13] M. Wakisaka, K. Suto, J. Inukai, K. Itaya, Langmuir 18 (2002) 729. [14] J. Inukai, M. Wakisaka, K. Itaya, Jpn. J. Appl. Phys. 43 (2004) 4549. [15] E. Zeigerson, E. Gilead, J. Electroanal. Chem. 28 (1970) 421. [16] M.P. Soriaga, A.T. Hubbard, J. Am. Chem. Soc. 104 (1982) 3937. [17] D. Ren, A.T. Hubbard, J. Colloid Interface Sci. 209 (1999) 435. [18] F. Lu, G.N. Salaita, L. Laguren-Davidson, D.A. Stern, E. Wellner, D.G. Frank, N. Batina, D.C. Zapien, N. Walton, A.T. Hubbard, Langmuir 4 (1988) 637. [19] N. Batina, D.G. Frank, J.Y. Gui, B.E. Kahn, C.-H. Lin, F. Lu, J.W. McCargar, C.N. Salaita, D.A. Stern, D.C. Zapien, A.T. Hubbard, Electrochim. Acta 34 (1989) 1031. [20] J.Y. Gui, B.E. Kahn, L. Laguren-Davidson, C.-H. Lin, F. Lu, G.N. Salaita, D.A. Stern, A.T. Hubbard, Langmuir 5 (1989) 819. [21] R.J. Wilson, K.E. Johnson, D.D. Chambliss, B. Melior, Langmuir 9 (1993) 3478. [22] J.E. Soto, Y.-G. Kim, M.P. Soriaga, Electrochem. Commun. 1 (1999) 135. [23] J. Inukai, M. Wakisaka, M. Yamagishi, K. Itaya, J. Electrochem. Soc. in press. [24] J. Inukai, M. Wakisaka, M. Yamagishi, K. Itaya, Langmuir 20 (2004) 7507. [25] J. Wider, T. Greber, E. Wetli, T.J. Kreutz, P. Schwaller, J. Osterwalder, Surf. Sci. 417 (1998) 301. [26] Y.-G. Kim, S.-L. Yau, K. Itaya, Langmuir 15 (1999) 7810.
An STM study on the growth process of vapor-deposited hydroquinone adlayers on Rh(1 1 1) and Pt(1 1 1) Junji Inukai
a,b
, Mitsuru Wakisaka b, Kingo Itaya
b,c,*
a
b
NICHe, Tohoku University, 6-6-10 Aoba, Sendai 980-8579, Japan Department of Applied Chemistry, Faculty of Engineering, Tohoku University, 6-6-04 Aoba, Sendai 980-8579, Japan c CREST, JST, 4-1-8 Kawaguchi, Saitama 332-0012, Japan Received 10 August 2004; in final form 1 October 2004
Abstract Hydroquinone molecules were allowed to adsorb on Rh(1 1 1) and Pt(1 1 1) by vapor deposition. The strong interaction between p p two adjacent hydroxyl groups of neighboring molecules led to the formation of commensurate ( 7 · 7)R19.1 and incommensup p rate ( 2.56 · 2.56)R16 adlayer phases on Rh(1 1 1) and Pt(1 1 1), respectively. The formation processes of HQ adlayers on Rh(1 1 1) and Pt(1 1 1) were very different from each other, possibly due to the difference in lattice parameters of the two metal surfaces. 2004 Elsevier B.V. All rights reserved.
1. Introduction The adsorption of organic molecules on metal surfaces is a major subject of modern interfacial chemistry [1–3]. We previously investigated the adsorption of aromatic molecules such as benzene, naphthalene, anthracene, and coronene on well-defined Rh(1 1 1) [4–7], Pt(1 1 1) [4–6], Cu(1 1 1) [6,8], and Au(1 1 1)[9,10] surfaces by using scanning tunneling microscopy (STM) [6]. Recently, we showed that both intermolecular and molecule–substrate interactions play important roles in the formation of ordered adlayers of benzene derivatives. We investigated the adlayers of methyl-substituted benzene derivatives, such as toluene, o-xylene, p-xylene, and mesitylene, formed on Rh(1 1 1) in HF solution [11,12]. For p-xylene, we also investigate its adlayers on Rh(1 1 1) and Pt(1 1 1) formed by vapor deposition [13,14]. Although the atomic distances on Rh(1 1 1) and Pt(1 1 1) surfaces are different, namely 0.269 and *
Corresponding author. Fax: +81 22 214 5380. E-mail address: [email protected] (K. Itaya).
0009-2614/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2004.10.023
0.278 p nm, respectively, the same adlayer phase of c(2 3 · 4)rect was observed on both Rh(1 1 1) and Pt(1 1 1) in solution and vacuum [14], indicating that the intermolecular interaction between p-xylene molecules is small. Hydroquinone (HQ) is a well-studied molecule, which undergoes electrochemical oxidation/reduction reactions in solution [15]. Hubbard and co-workers investigated the adlayers of HQ on Pt electrodes by employing thin-layer electrochemical cells [3,16,17], low energy electron diffraction (LEED) [3,18,19], Auger electron spectroscopy [3,18,19], and high-resolution electron energy loss spectroscopy (HREELS) [3,18–20]. Wilson et al. [21] performed STM studies of HQ by using a UHV compatible cell, wherein a drop of ultrapure water containing HQ was evaporated from a Pt(1 1 1) surface in an Ar atmosphere. Soto et al. [22] investigated the adlayers of HQ on Pd(1 1 1) and Pd(1 0 0) electrode surfaces by tandem electrochemistry, HREELS, and in situ STM. Those reports showed that adsorbed HQ molecules formed monolayers with an orientation parallel to the Pt and Pd single-crystal surfaces,
374
J. Inukai et al. / Chemical Physics Letters 399 (2004) 373–377
and that the hydrogen in hydroxyl group was dissociated from the molecule. Very recently, we reported the surface structures of the adlayers of HQ on p Rh(1 p 1 1) [23] and Pt(1 1 1) [24] to be commensurate ( 7 · 7)R19.1 and incommensurate (2.56 · 2.56)R16, respectively, regardless of whether they were formed in solution or in vacuum. Surprisingly, although the surface symmetries were different as stated above, the adlayer structures of HQ on both metal surfaces were identical [23,24]. Figs. 1a and b show structure models for the adlayers of HQ on Rh(1 1 1) and Pt(1 1 1), respectively [23,24]. In these models, the hydrogen in hydroxyl group is depicted to be dissociated from the molecule as reported previously [3,18–22]. The HQ molecules are adsorbed horizontally with respect to the Rh(1 1 1) and Pt(1 1 1) planes. On
Rh(1 1 1), HQ molecules are located at threefold sites so that all oxygen atoms are allowed to be adsorbed at threefold sites [25]. The strong interaction between two adjacent hydroxyl groups of neighboring HQ molecules with sp3 oxygen orbitals leads to the formation of the zigzag configuration of molecular binding (Fig. 1) [23,24]. The of HQ on Rh(1 1 1) and Pt(1 1 1) with p coverages p ( 7 · 7)R19.1 and (2.56 · 2.56)R16 phases were calculated to be 0.143 and 0.153, respectively, but the packing density of HQ on Rh(1 1 1) and Pt(1 1 1) coincided to be 0.379 nmol cm2 on both surfaces because of the identical adlayer structures [23,24]. In this study, we carried out an STM investigation of the formation process of HQ adlayers on Rh(1 1 1) and Pt(1 1 1) by vapor deposition. Perturbations were observed in the bonding between HQ molecules on Rh(1 1 1) and Pt(1 1 1).
2. Experimental Single-crystal beads of Rh and Pt, 3 mm in diameter, were made by crystallization of molten balls formed at the end of Rh and Pt wires in a hydrogen–oxygen flame [6]. The laser beam deflection method was used to determine the orientation of the single-crystal beads to expose the (1 1 1) planes, which were then mechanically polished with successively finer-grade diamond pastes down to 0.05 lm with an accuracy in the angle of <0.1. The vapor deposition of HQ (reagent grade, Kanto Chemical Co.) on Rh(1 1 1) and Pt(1 1 1) was carried out in a vacuum chamber, and the specimens produced were subsequently investigated by LEED and STM [23,24].
3. Results and discussion
Fig. 1. Structural models of ordered HQ adlayers on Rh(1 1 1) (a) and p p Pt(1 1 1) (b). Commensurate ( 7 · 7)R19.1 and incommensurate (2.56 · 2.56)R16 adlayer structures are shown in (a) and (b).
In our previous work, we continuously exposed Rh(1 1 1) to p-xylene in a vacuum chamber during the STM observation to follow time-dependent processes during the growth of ordered phases [13]. During the continuous dosing, adsorbed p-xylene molecules were very mobile [13]. The formation of the disordered adlayer was found at the first stage of adsorption of p-xylene [14]. As described below, the results obtained with HQ were in distinct contrast to those obtained with pxylene. Fig. 2 shows STM images of HQ molecules on Rh(1 1 1) after the vapor deposition. As can be seen in Fig. 2a, HQ molecules interact with each other even at a low coverage of approximately 0.05, in contrast to the case of p-xylene p [13,14]: linear chains of HQ molecules run in the 7 direction. The width of each chain is seen to correspond to 1–3 molecules, showing the existence of lateral interaction between the molecules.
J. Inukai et al. / Chemical Physics Letters 399 (2004) 373–377
375
Fig. 3 shows the STM images revealing the growth process of the HQ adlayer on Pt(1 1 1). At the first stage of adsorption with a coverage of 0.02 (Fig. 3a), the molecules are clearly seen to interact with each other as was observed on Rh(1 1 1) (Fig. 2). However, the growth process was different from that on Rh(1 1 1). In the case on Rh(1 1 1), p straight chains of molecules were seen to run in the 7 direction (Fig. 2a), whereas on Pt(1 1 1), the molecules form winding chains (Fig. 3a) [21]. On Rh(1 1 1), the molecular chains do not cross each other
Fig. 2. STM images of vapor-deposited HQ molecules on Rh(1 1 1). Solid arrows indicate close-packed directions of the Rh(1 1 1) subp strate. The dashed arrows indicate the 7 direction. (a) After exposure 7 to HQ at 5 · 10 Torr for 1 s (0.5 L). Scan area = 25 · 25 nm2. Tip bias = 0.50 V. Tunneling current = 1.0 nA. (b) After exposure at 5 · 107 Torr for 5 s (2.5 L). Scan area = 40 · 40 nm2. Tip bias = 0.35 V. Tunneling current = 1.3 nA.
The length of each chain is 10–20 nm. The molecular chains are all straight, exhibiting no curvatures. Fig. 2b shows an STM image of the monolayer of HQ on Rh(1 1 1). HQ molecules have been proposed to be adsorbed at threefold sites as depicted in Fig. 1a [23]. The observation with LEED clearly showed a p p ( 7 · 7)R19.1 pattern on the surface [23]. The ordered domains are narrow, 10–20 nm in length, apparently reflecting the growth process of the HQ layer on Rh(1 1 1) (Fig. 2a). At domain boundaries, some HQ molecules look brighter, probably because of the adsorption of those molecules on the monolayer of HQ. Interestingly, the domains of HQ on Rh(1 1 1) (Fig. 2b) are similar in shape to those of terephthalic acid on Pt(1 1 1) [26], which formed hydrogen bonding between the molecules.
Fig. 3. STM images of vapor-deposited HQ molecules on Pt(1 1 1). Solid arrows indicate close-packed directions of the Pt(1 1 1) substrate. (a) After exposure to HQ at 5 · 107 Torr for 2 s (1 L). Scan area = 25 · 25 nm2. Tip bias = 1.50 V. Tunneling current = 0.5 nA. (b) After exposure at 5 · 107 Torr for 6 s (3 L). Scan area = 20 · 20 nm2. Tip bias = 0.05 V. Tunneling current = 4.0 nA. (c) After exposure at 5 · 107 Torr for 12 s (6 L). Scan area = 25 · 25 nm2. Tip bias = 0.35 V. Tunneling current = 2.0 nA.
376
J. Inukai et al. / Chemical Physics Letters 399 (2004) 373–377
(Fig. 2a), while on Pt(1 1 1), some crossings of molecular chains are observed (Fig. 3a). As the coverage by HQ molecules on Pt(1 1 1) increased, the molecular chains began to align themselves in the direction rotated by approximately 16 with respect to the Pt atomic rows (Figs. 1b and 3b). When the full coverage was reached, the formation of a (2.56 · 2.56)R16 phase was confirmed by LEED [24]. Some HQ molecules, adsorbed on the monolayer, also look brighter on Pt(1 1 1). As can be seen in Fig. 3c, the domains of HQ were very small on Pt(1 1 1), namely 5 nm. On Rh(1 1 1), domain boundaries were very clearly developed (Fig. 2b), whereas on Pt(1 1 1), it is difficult even to distinguish domain boundaries, because HQ molecules at boundaries are interconnected (Fig. 3c). Even at the first stage of adsorption, HQ molecules were observed to interact each other both on Rh(1 1 1) and Pt(1 1 1), forming chains, which was not observed for p-xylene molecules: each p-xylene molecules were isolated at low coverages, showing little, if any, attractive interaction [14]. By using HREELS, Hubbard and co-workers [3,18–20] reported that on a Pt(1 1 1) electrode, the hydrogen atom in a hydroxyl group was dissociated from the molecule, because the OH band was absent. If this holds, the dissociated hydrogen atoms on Rh(1 1 1) and Pt(1 1 1) might be adsorbed between the two oxygen atoms in the neighboring HQ molecules. This would form a hydrogen bond, O H O, between the neighboring HQ molecules. By LEED, Hubbard and coworkers [3,18,19] reported a (3 · 3) phase for the HQ adlayer on Pt(1 1 1) with a packing density of only 0.276 nmol cm2, whereas we formed a (2.56 · 2.56)R16 phase with a packing density of 0.379 nmol cm2. The two structures are very different, thus, in our experiments, there exists a possibility that some hydroxyl groups might remain at the surface species on Rh(1 1 1) and Pt(1 1 1) to form hydrogen bonds. Wilson et al. [21] suggested that the formation of HQ chains may result from hydrogen bonding of the phenolic H in solution prior to dissociation. Our study showed that the identical chains are formed not only in solution but also in vacuum, which cannot be explained by the mechanism proposed by Wilson et al. The difference between the HQ adlayers on Rh(1 1 1) and Pt(1 1 1) can be geometrically explained in terms of the lattice parameters, 0.269 nm on Rh(1 1 1) and 0.278 nm on Pt(1 1 1). The lattice parameter of HQ adlayer must closely match with that of Rh(1 1 1) in order for the HQ molecules to be naturally situated at commensup rate sites: the HQ chains and domains grow in the 7 direction (Fig. 2). On the other hand, the lattice parameter on Pt(1 1 1) is slightly larger for the HQ adlayer, and therefore, HQ molecules cannot be situated at commensurate sites. In the process of forming a bond for a new HQ molecule on Pt(1 1 1), it will search for an incommensurate, but energetically more favorable, sur-
Fig. 4. High-resolution STM image of HQ monolayer on Pt(1 1 1) after exposure to HQ at 5 · 107 Torr for 12 s (6 L). Scan area = 5 · 5 nm2. Tip bias = 0.35 V. Tunneling current = 2.0 nA. Molecular structures of HQ are superimposed.
face site. Consequently, winding molecular chains are expected to grow (Fig. 3a). When the HQ coverage increases on Pt(1 1 1), the lateral interaction between adjacent HQ molecules causes the formation of the ordered, incommensurate adlayer structure (Figs. 3b and c). The formation process of the HQ adlayers shows that the interaction between molecules and Rh(1 1 1) and Pt(1 1 1) substrates are not very strong, but strong enough to produce differences on the two substrates. Fig. 4 shows a high-resolution STM image of a HQ adlayer on Pt(1 1 1) with HQ molecular structures superimposed. Molecules of HQ, each of which has a phenyl ring at the center and two hydroxyl groups, one on each side, are seen to form a bond between neighboring hydroxyl groups. The chains of HQ are seen to be bent as described above, which was never observed on Rh(1 1 1) (Fig. 2). Interestingly, not only two but even three hydroxyl groups are seen to interconnect with each other. In the case of HQ on Rh(1 1 1), the interconnection with three hydroxyl groups could not be formed because the molecular chains were never bent (Fig. 2). On the other hand, chains are bent on Pt(1 1 1), and the bending point with an sp3 oxygen orbital at the apex could serve as the interconnection with three molecules. The bonding between HQ molecules was found to be perturbed on different surfaces with different lattice parameters, due to the difference in organizing process of the adlayers.
4. Conclusions Molecules of HQ were allowed to adsorb on Rh(1 1 1) and Pt(1 1 1) by vapor deposition. The HQ
J. Inukai et al. / Chemical Physics Letters 399 (2004) 373–377
molecules were found to form commensurate p p ( 7 · 7)R19.1 and incommensurate (2.56 · 2.56) R16 adlayer structures on Rh(1 1 1) and Pt(1 1 1), respectively. The formation processes of HQ adlayers on the two surfaces were different from each other probably due to the difference in substrate lattice parameters. Acknowledgements This work was supported partially by the COE project, ÔGiant molecules and Complex Systems, 2004,Õ from the Ministry of Education, Culture, Sports, Science and Technology, Japan, and by Core Research for Evolutional Science and Technology organized by the Japan Science and Technology Corporation. The authors thank Dr. Y. Okinaka for his help in writing this manuscript.
References [1] J. Lipkowski, P.N. Ross (Eds.), Adsorption of Molecules at Metal Electrodes, VCH, New York, 1992. [2] A. Wieckowski (Ed.), Interfacial Electrochemistry, Marcel Dekker, New York, 1999. [3] A.T. Hubbard, Chem. Rev. 88 (1988) 633. [4] S.-L. Yau, Y.-G. Kim, K. Itaya, J. Am. Chem. Soc. 118 (1996) 7795. [5] S.-L. Yau, Y.-G. Kim, K. Itaya, J. Phys. Chem. B 101 (1997) 3547. [6] K. Itaya, Prog. Surf. Sci. 58 (1998) 121.
377
[7] J. Inukai, M. Wakisaka, K. Itaya, Jpn. J. Appl. Phys. 43 (2004) 4554. [8] L.-J. Wan, K. Itaya, Langmuir 13 (1997) 7173. [9] S. Yoshimoto, R. Narita, K. Itaya, Chem. Lett. (2002) 356. [10] S. Yoshimoto, R. Narita, M. Wakisaka, K. Itaya, J. Electroanal. Chem. 532 (2002) 331. [11] K. Suto, M. Wakisaka, M. Yamagishi, L.-J. Wan, J. Inukai, K. Itaya, Langmuir 16 (2000) 9368. [12] K. Suto, J. Inukai, K. Itaya, J. Electroanal. Chem. 550–551 (2003) 105. [13] M. Wakisaka, K. Suto, J. Inukai, K. Itaya, Langmuir 18 (2002) 729. [14] J. Inukai, M. Wakisaka, K. Itaya, Jpn. J. Appl. Phys. 43 (2004) 4549. [15] E. Zeigerson, E. Gilead, J. Electroanal. Chem. 28 (1970) 421. [16] M.P. Soriaga, A.T. Hubbard, J. Am. Chem. Soc. 104 (1982) 3937. [17] D. Ren, A.T. Hubbard, J. Colloid Interface Sci. 209 (1999) 435. [18] F. Lu, G.N. Salaita, L. Laguren-Davidson, D.A. Stern, E. Wellner, D.G. Frank, N. Batina, D.C. Zapien, N. Walton, A.T. Hubbard, Langmuir 4 (1988) 637. [19] N. Batina, D.G. Frank, J.Y. Gui, B.E. Kahn, C.-H. Lin, F. Lu, J.W. McCargar, C.N. Salaita, D.A. Stern, D.C. Zapien, A.T. Hubbard, Electrochim. Acta 34 (1989) 1031. [20] J.Y. Gui, B.E. Kahn, L. Laguren-Davidson, C.-H. Lin, F. Lu, G.N. Salaita, D.A. Stern, A.T. Hubbard, Langmuir 5 (1989) 819. [21] R.J. Wilson, K.E. Johnson, D.D. Chambliss, B. Melior, Langmuir 9 (1993) 3478. [22] J.E. Soto, Y.-G. Kim, M.P. Soriaga, Electrochem. Commun. 1 (1999) 135. [23] J. Inukai, M. Wakisaka, M. Yamagishi, K. Itaya, J. Electrochem. Soc. in press. [24] J. Inukai, M. Wakisaka, M. Yamagishi, K. Itaya, Langmuir 20 (2004) 7507. [25] J. Wider, T. Greber, E. Wetli, T.J. Kreutz, P. Schwaller, J. Osterwalder, Surf. Sci. 417 (1998) 301. [26] Y.-G. Kim, S.-L. Yau, K. Itaya, Langmuir 15 (1999) 7810.