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2008, 112, 7540–7543 Published on Web 04/24/2008
Brass Surface Nanochemistry: The Role of Alloying Cu with Zn Fre´de´ric Wiame,* Bekir Salgin, Jolanta Swiatowska-Mrowiecka, Vincent Maurice,* and Philippe Marcus* Laboratoire de Physico-Chimie des Surfaces, ENSCP/CNRS (UMR 7045), Ecole Nationale Supe´rieure de Chimie de Paris, 11 rue Pierre et Marie Curie, 75231 Paris Cedex 05 ReceiVed: February 28, 2008; ReVised Manuscript ReceiVed: March 31, 2008
For the first time the surface of brass (Cu-Zn alloy) and its chemistry have been characterized by scanning tunneling microscopy/spectroscopy at the subnanometer (atomic) scale. The comparative results obtained on Cu70Zn30(111) and Cu(111) single-crystal surfaces evidence the random distribution of Zn atoms substituted to Cu atoms at the extreme surface of brass, the Zn-depleted concentration in the topmost layer of the alloy and the associated modifications of the surface density of states. The initial stages of interaction with oxygen and sulfur highlight the role of Zn in the surface chemistry. The reactivity becomes governed by the chemical defects (Zn atoms) introduced by alloying. The nucleation mechanism and the nature of the surface oxide compounds are modified. Surface reconstruction is promoted in the reaction with sulfur, and S vacancies are introduced in the surface sulfide compounds. The data are relevant for the detailed understanding of the effect of alloying a noble or seminoble metal matrix with a more reactive chemical element on the growth mechanisms of adhesion conversion layers and on corrosion mechanisms by dealloying. The detailed understanding of the role of alloying elements added to a metal is a challenging issue to control surface or interface properties and to counteract detrimental effects. Brass, an alloy made of copper and zinc used in many technical applications, is a key component of metal-rubber composite systems in industrially important technologies, because it provides the adhesion properties between rubber and the reinforcing metallic cords in car and truck tires. The effective bonding of rubber to the brass-coated steel reinforcing cord is critical to lifetime and performance, and the rubber-brass interfacial morphology and the mechanisms of adhesion have been extensively discussed.1–4 A better understanding and control of the chemical interaction of rubber with the brass surface at the nanoscale could lead to significant progress in the manufacturing process. The surface of brass has been the subject of much interest in the past.5–12 However, although it appears as crucial, no studies including the surface atomic structure of the alloy at the solid/ gas interface have been reported. Scanning tunneling microscopy (STM) results have been obtained in the case of Zn deposited on Cu(111) surfaces.13 In this case, the authors observed surface alloying, and the STM images suggested that the Zn atoms were randomly distributed into the Cu surface. However, such studies are unable to reproduce phenomena involving not only the surface but also the near-surface of bulk Cu-Zn alloys and that are of crucial importance for the growth mechanisms of conversion layers1–4,12 or dealloying mechanisms in corrosion.14–19 Here, we show for the first time at the subnanometer scale how the presence of zinc in a copper bulk matrix modifies the structure at the brass surface, and demonstrate the effect of zinc on * To whom correspondence should be addressed. E-mail: (F.W.)
[email protected]; (V.M.)
[email protected]; (P.M.)
[email protected]. Phone: +33 1 44276736. Fax: +33 1 46340753.
10.1021/jp801739b CCC: $40.75
the surface reactivity toward oxygen and sulfur, two chemical elements essential to the rubber-brass interfacial chemistry. The experiments were performed in an ultrahigh vacuum (UHV) system (base pressure below 1 × 10-10 mbar) equipped with scanning tunneling microscopy/spectroscopy (STM/STS, Omicron STM1 with SCALA system) and with facilities for surface chemical control by Auger electron spectroscopy (AES, CMA OPC105 from Riber) and structural control by low energy electron diffraction (LEED, OPD304 from Riber) as well as argon ion sputtering, annealing, and gas dosing of the sample. All STM images were recorded at room temperature in constant current mode (topographic mode). No filtering was used. Background plane subtraction was applied and linear drift correction was performed when needed. STS measurements were performed in the current imaging tunneling spectroscopy mode in which the I(V) curves are recorded at a fixed tip-sample distance (feedback loop disabled) for each pixel of an image recorded at preset sample bias and tunneling current. A major difficulty in the preparation of well-controlled brass surfaces is dezincification (i.e., the loss of Zn) that occurs at relatively low temperature via sublimation. In our study, the surface of a Cu70Zn30 single-crystal (Cu/Zn atomic ratio of 70/ 30, alpha-brass) oriented along the (111) planes of its facecentered cubic (fcc) structure was prepared by mechanical polishing with diamond spray with a final grading of 0.25 µm and then annealed at 250 °C for two hours under a flow of ultrapure (6N) hydrogen at atmospheric pressure. Higher annealing temperatures were avoided as they induce noticeable dezincification of the sample. In situ the surface was further prepared by multiple cycles of mild argon ion bombardment (p(Ar) ) 1 × 10-5 mbar; 500 eV energy; 1 µA sample current) and annealing (120 °C) until a sharp (1 × 1) LEED pattern was observed and no contamination was detected in the AES 2008 American Chemical Society
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J. Phys. Chem. C, Vol. 112, No. 20, 2008 7541
Figure 2. (a) Tunneling current and (b) normalized differential conductance curves averaged from I(V) spectra recorded at each pixel of an STM image obtained at 0.3 V and 3 nA on the Cu70Zn30(111) surface.
Figure 1. STM images showing (a,b) high-resolution (atomic) chemical contrast at the clean and (111)-oriented surface of a Cu70Zn30 singlecrystal: (a) 0.1 V sample bias voltage, 3 nA tunneling current; (b) 0.01 V, 3 nA. (c) No atomic chemical contrast for pure Cu(111) (0.01 V, 4 nA); (d) Friedel oscillations generated by Zn atoms and step edges and forming a disordered pattern on the terraces for Cu70Zn30(111) (0.7 V, 0.2 nA). Dotted lines mark the step edges. The pointers indicate the Zn-induced perturbations on the terraces.
spectrum. Care was taken to limit the annealing temperature because under UHV increasing the temperature above 120 °C induces a very fast dezincification over several hundreds of nanometers in depth, as evidenced by SEM and confirmed by the sample’s color change. This has the drawback of timeconsuming preparation. The dosing experiments were performed by background filling the chamber to p(O2) ) 1 × 10-7 mbar or p(H2S) ) 5 × 10-7 mbar. As prepared, the clean brass surface presented a hill and valley topography at the nanoscale. The terraces, separated by monatomic steps (0.21 nm high), were atomically flat and up to several tens of nanometers in width. Figures 1a,b show atomically resolved STM images obtained on the terraces of the clean Cu70Zn30(111) brass surface. An hexagonal lattice consistent with the (111) orientation of the fcc structure of the alloy is revealed as observed on pure Cu(111) (Figure 1c). However, bright spots (some are pointed), appearing from 0.01 to 0.03 nm higher than the matrix atoms, are observed on the brass surface but not on pure copper. Their presence is assigned to the Zn atoms revealed by atomic chemical contrast. This is the first direct observation of atomic chemical contrast at the surface of Cu-Zn alloys. Such contrast has been previously observed only with high melting point alloys.20–22 The chemical contrast dramatically depends on the electronic structure of the imaging tip and is not directly related to the resolution of the image. Indeed, chemical contrast could be observed without atomic resolution, and reversely atomic resolution could be obtained without chemical contrast. This indicates that the measured chemical contrast cannot by explained by the undisturbed surface electronic structure alone.23 The presence of some contaminants at the apex of the tip may, for example, increase the recorded corrugation. A major outcome of all the high-resolution images acquired on the brass surface is the direct evidence of the random
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Figure 3. STM images showing (a) Zn-driven random nucleation on the terraces of a Cu70Zn30(111) surface exposed to 20 L of oxygen at room temperature and low pressure (0.7 V, 0.2 nA); (b) structural defect-driven nucleation at the step edges of a Cu(111) surface exposed to 400 L of oxygen at room temperature and low pressure (0.7 V, 0.05 nA). Dotted lines mark the step edges; “ox” and “ads” indicate oxide nuclei and O adatoms, respectively.
distribution of the Zn atoms substituted to Cu in the alloy matrix and the absence of clustering of the Zn atoms in agreement with the ideal solid solution model. Moreover, such images allow a direct measurement of the composition of the alloy at the extreme surface. The average Zn/Cu ratio obtained from several atomic resolution images is 15/85, which is in contrast with the 30/70 ratio in the bulk of the alloy. The Zn surface concentration is thus half the bulk concentration in agreement with previous conclusions from indirect spectroscopic measurements that could not discriminate the topmost layer.11 This surface dezincification is attributed to the effect of a preferential ion sputtering of the zinc during the argon ion bombardment pretreatment.7,9,11 It results at least partially from the smaller binding energy of the Zn to the alloy matrix, which is also consistent with the observed dezincification under annealing. Moreover, these results show that annealing the sample at 120 °C for several hours does not restore the bulk concentration at the surface. Enlarged images recorded at a larger tip-surface distance (obtained by selecting the tunneling conditions) revealed oscillations (0.04 nm corrugation, pointed by the arrow) forming a disordered pattern and fully covering the terraces (Figure 1d). The step edges (marked by dotted lines) also appear higher (0.03 nm) than the delimited upper terraces. These features result from local perturbations, known as Friedel oscillations,24 in the surface charge density of the free electrons of the alloy generated by
Letters
Figure 4. STM images showing (a) the defective 7 superstructure formed on the Cu70Zn30(111) surface exposed to 15 L of H2S at room temperature and low pressure (0.3 V, 3 nA); (b) the perfect 7 superstructure formed on the Cu(111) surface saturated with 72 L of H2S at room temperature and low pressure (0.2 V, 1 nA). The 7 unit cell is marked. Dotted squares indicate S vacancies. The pointers indicate the Zn-modified density of state at S adatoms. The inset in panel a shows a better resolved unit cell.
the perturbing elements that are the Zn atoms and the step edges. The resulting disordered pattern further evidences the random distribution of the Zn atoms generating the local oscillations on the terraces of the alloy characterized in this case at the nanoscale. Figure 2 shows the surface density of states induced by alloying Cu with Zn. The variations of the tunneling current and normalized differential conductance, proportional in first approximation to the surface density of states, are shown as a function of the applied voltage for the brass surface. The density of states is symmetrical around 0 V except for two states centered at -2.2 and -1.8 V (marked in gray). The two main features are centered at -1 and 1 V with maxima at -0.7 and 0.7 V. Another feature is present at 0 V and local minima are observed at -0.4 and 0.4 V. No other states could be observed between -7 and 7 V at this tunneling distance. Various tips gave the same results showing a highly reproducible structure. Varying the tip-to-surface distance (defined by the tunneling conditions preset before spectroscopic acquisition) was observed to have a significant influence since the structure in Figure 2 was only observed at very close distance (i.e., tunneling currents higher than 1 nA). At larger distances, a structure similar to that observed for Cu(111),25–27 and typical of a free electron gas was obtained, showing that the modified electronic structure results from the alloying of Cu with Zn in the topmost plane of the alloy.
Letters In the following, we present two striking examples of the modifications of the nanochemistry of the surface induced by alloying Cu with Zn. The first example, given in Figure 3, is the oxide nucleation at the subnanometer scale upon exposure of brass and copper surfaces to gaseous oxygen. After an exposure of 20 L at room temperature (Figure 3a), the brass surface showed the random distribution of two-dimensional (2D) islands (marked “ox”) corresponding to oxide nuclei having an apparent width of ∼1 nm. Note the absence of preferential nucleation at the step edges (marked by dotted lines) showing that nucleation is homogeneous. This nucleation mechanism differs markedly from that observed on pure Cu(111) surfaces having the same structure (Figure 3b). In this case and for similar conditions of exposure to gaseous oxygen, the nucleation of the 2D surface oxide occurs exclusively at the step edges (marked by dotted lines) delimiting the terraces and the vacancy islands. Only adsorbed oxygen atoms (marked “ads”), either isolated or forming clusters,27 are observed on the defect-free terraces. Furthermore, the apparent height of the oxide islands on brass (∼+0.2 nm) and copper (∼-0.1 nm) is different (independently of the tunneling bias conditions), which is consistent with the growth of a chemically different oxide, ZnO on brass12,28 and Cu2O on copper.27 These differences emphasize the role of the chemical atomic heterogeneities on the surface reactivity of the alloy. Zn atoms act as chemical defects on Cu-Zn surfaces, whereas on pure Cu, only structural defects play a role. As a result oxide nucleation takes place on terraces on Cu-Zn, and at step edges on Cu. The second example is the modification of the structure of the 2D surface layer formed by reaction of the alloy surface with sulfur. An identical superstructure, the so-called 7 ((7 × 7)R19.1° in full notation), is formed on both Cu and Cu-Zn substrates (Figure 4). Its unit cell contains 3 S atoms and 7 metal atoms per substrate plane. Most importantly, it involves the reconstruction of the metal topmost layer due to the strong sulfur-to-metal bonding to form a 2D surface sulfide precursor, as previously discussed for pure Cu(111).29–32 A major effect of Zn is to promote the growth of islands with the 7 superstructure at low surface coverage, whereas without Zn full saturation is required to form this structure at room temperature. This shows the effect of alloying on the promotion of the surface reconstruction and the resulting Zn-induced modified nucleation mechanism for surface sulfides as in the case of surface oxides. In addition, numerous S vacancies (some are marked in Figure 4a) characterize the 7 superstructure on brass, showing the defective nature of the adlayer as opposed to the defect-free adlayer formed on pure copper. These defects could result from the presence of Zn substituted to Cu in the adsorption sites of the missing S atoms. The presence of Zn is confirmed by its typical effect on the contrast at high resolution because bright spots (marked by pointers in Figure 4a) can also be observed depending on the tunneling conditions. The density of these bright spots is consistent with the atomic concentration of Zn because the value ∼15% can allow the presence of 1 Zn atom per 7 unit cell (1/7 ) 14.2%). They are assigned to S adatoms where the density of states is modified by the presence of Zn in the vicinity. In summary, these subnanometer scale data directly evidence for the first time the random distribution of Zn atoms substituted to Cu atoms at the extreme surface of brass, their concentration, and the associated modifications of the surface density of states. From structural defect-driven at step edges on a pure metallic matrix, the surface reactivity becomes governed by the chemical defects (Zn atoms) introduced by alloying. The nucleation
J. Phys. Chem. C, Vol. 112, No. 20, 2008 7543 mechanism and the nature of the surface oxide compounds formed by reaction with oxygen are modified. Surface reconstruction is promoted in the reaction with sulfur, and S vacancies are introduced in the surface sulfide compounds. Such data highlight the role of Zn in the surface nanochemistry of brass and more generally are relevant for the understanding of the effect of alloying a noble or seminoble metal matrix with a more reactive chemical element. This is a key issue for the development of technological interfaces of improved performances and lifetime. Acknowledgment. We acknowledge the support of the Centre de Technologie Michelin at Clermont-Ferrand, France. References and Notes (1) Van Ooij, W. J. Rubber Chem. Technol. 1979, 52, 605–675. (2) Van Ooij, W. J. Rubber-to-Brass Bonding. In Handbook of Rubber Bonding; Crowther, B., Ed.; RAPRA Technologies: Shrewsbury, U.K., 2001; Chapter 6. (3) Fulton, W. S. Rubber Chem. Technol. 2005, 78, 426–457. (4) Fulton, W. S.; Sykes, D. E.; Smyth, G. C. Appl. Surf. Sci. 2006, 252, 7074–7077. (5) Van Ooij, W. J. Surf. Sci. 1977, 68, 1–9. (6) Pelletier, J. B.; Toesca, S.; Colson, J. C. Appl. Surf. Sci. 1983, 14, 375–381. (7) Ferron, J.; De Bernardez, L. S.; Goldberg, E. C.; Buitrago, R. H. Appl. Surf. Sci. 1983, 17, 241–248. (8) Maroie, S.; Haemers, G.; Verbist, J. J. Appl. Surf. Sci. 1984, 17, 463–476. (9) Darque-Ceretti, E.; Delamare, F.; Blaise, G. Surf. Interface Anal. 1985, 7, 141–149. (10) Deroubaix, G.; Marcus, P. Surf. Interface Anal. 1992, 18, 39–46. (11) Hammer, G. E. J. Vac. Sci. Technol., A 1999, 17, 895–898. (12) Wiame, F.; Maurice, V.; Marcus, P. Surf. Sci. 2007, 601, 4402– 4406. (13) Sano, M.; Adaniya, T.; Fujitani, T.; Nakamura, J. J. Phys. Chem. B 2002, 106, 7627–7633. (14) Erlebacher, J.; Aziz, M. J.; Karma, A.; Dimitrov, N.; Sieradzki, K. Nature 2001, 410, 450–453. (15) Morales, J.; Esparza, P.; Vazquez, L.; Salvarezza, R. C.; Arvia, A. J. Langmuir 1996, 12, 500–507. (16) Martín, H.; Carro, P.; Herna´ndez Creus, A.; Morales, J.; Ferna´ndez, G.; Esparza, P.; Gonza´lez, S.; Salvarezza, R. C.; Arvia, A. J. J. Phys. Chem. B 2002, 104, 8239–8237. (17) Yeh, F. H.; Tai, C. C.; Huang, J. F.; Sun, I. W. J. Phys. Chem. B 2006, 110, 5215–5222. (18) Renner, F. U.; Stierle, A.; Dosch, H.; Kolb, D. M.; Lee, T.-L.; Zegenhagen, J. Nature 2006, 439, 707–710. (19) Jia, F. L.; Yu, C. F.; Deng, K. J.; Zhang, L. Z. J. Phys. Chem. C 2007, 111, 8424–8431. (20) Schmid, M.; Stadler, H.; Varga, P. Phys. ReV. Lett. 1993, 70, 1441– 1444. (21) Wouda, P. T.; Nieuwenhuys, B. E.; Schmid, M.; Varga, P. Surf. Sci. 1996, 359, 17–22. (22) Ondracek, M.; Maca, F.; Kudrnovsky, J.; Redinger, J.; Biedermann, A.; Fritscher, C.; Schmid, M.; Varga, P. Phys. ReV. B 2006, 74, 235437. (23) Hofer, W. A.; Ritz, G.; Hebenstreit, W.; Schmid, M.; Varga, P.; Redinger, J.; Podloucky, R. Surf. Sci. 1998, 405, L514–L519. (24) Friedel, J. NuoVo Cimento 1958, 7, 287–311. (25) Crommie, M. F.; Lutz, C. P.; Eigler, D. M. Nature 1993, 363, 524– 527. (26) Vazquez de Parga, A. L.; Hernan, O. S.; Miranda, R.; Levy Yeyati, A.; Mingo, N.; Martin-Rodero, A.; Flores, F. Phys. ReV. Lett. 1998, 80, 357–360. (27) Wiame, F.; Maurice, V.; Marcus, P. Surf. Sci. 2007, 601, 1193– 1204. (28) Cokoja, M.; Parata, H.; Schro¨ter, M. K.; Birkner, A.; van den Berg, M. W. E.; Klementiev, K. V.; Gru¨nert, W.; Fischer, R. A. J. Mater. Chem. 2006, 18, 2420–2428. (29) Domange, J. L.; Oudar, J. Surf. Sci. 1968, 11, 124–142. (30) Prince, N. P.; Seymour, D. L.; Ashwin, M. J.; McConville, C. F.; Woodruff, D. P.; Jones, R. G. Surf. Sci. 1990, 230, 13–26. (31) Foss, M.; Feidenhans’l, R.; Nielsen, M.; Findeisen, E.; Buslap, T.; Johnson, R. L.; Besenbacher, F. Surf. Sci. 1997, 388, 5–14. (32) Saidy, M.; Mitchell, K. A. R. Surf. Sci. 1998, 441, 424–435.
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