Surface Decoration at the Atomic Scale Using a Molecular Pattern

Jun 12, 2009 - We report on copper adsorption through a self-ordered molecular pattern, namely cyanide-modified Pt(111), in a attempt to develop a ...
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Surface Decoration at the Atomic Scale Using a Molecular Pattern: Copper Adsorption on Cyanide-Modified Pt(111) Electrodes ´ ngel Cuesta* Marı´a Escudero, Jose´ F. Marco, and A Instituto de Quı´mica Fı´sica “Rocasolano”, CSIC, C. Serrano, 119, E-28006 Madrid, Spain ReceiVed: January 26, 2009; ReVised Manuscript ReceiVed: May 18, 2009

We report on copper adsorption through a self-ordered molecular pattern, namely cyanide-modified Pt(111), in a attempt to develop a procedure with which periodic metallic nanostructured deposits could be fabricated. We show that copper (either in ionic or metallic state) adsorbs on the surface of a cyanide-modified Pt(111) electrode following exactly the molecular pattern. Our work presents two novelties: (i) the attempt to use a molecular pattern to govern the deposition of a metal, and not the adsorption of an organic or organometallic molecule or supramolecule, and (ii) the line width of the pattern, of only one atomic diameter. Introduction The past decades have witnessed an increasing interest in the generation of nanometer-sized structures on surfaces, mainly driven by the need of the microelectronics, data storage and communication industries for ever smaller feature sizes. The advent, in the mid 80s, of the scanning probe microscopes (SPMs),1 mainly the scanning tunneling microscope (STM) and the atomic force microscope (AFM), made the modification of surfaces at the nanometer and atomic scale possible.2 The capability to easily create and control very high electric fields (of the order of 109 V m-1) at the solid-electrolyte interface makes electrochemical methods very promising for surface nanostructuring. In an electrochemical environment, nanostructuring of a surface can be achieved, for example, by preferential electrodeposition of a metal at monatomic step edges separating atomically flat terraces on highly ordered pyrolytic graphite (HOPG),3 or at the hexagonal close-packed (hcp) regions between the sets of soliton walls of the reconstructed Au(111) surface.4 The combination of both approaches, using STM as a nanotool in an electrochemical environment, was pioneered by Penner and co-workers, who used the tip of an STM to create surface defects, that then act as nucleation centers for the electrodeposition of a metal.5 A very elegant method of electrochemical nanoscale modification was reported by Kolb’s group,6 who could transfer Cu predeposited on the STM tip to the surface of a Au(111) electrode by carefully approaching to it the STM tip. In combination with a microprocessing unit, various patterns and cluster arrays (including geometric and artistic drawings and words) can be easily and reproducibly created. We aim to combine the versatility in structural motifs and the parallel character of lithographic methods with the accessibility and inexpensiveness of electrodeposition processes, and to fabricate periodic metallic nanostructured deposits with feature sizes of one atomic diameter, a limit up to now only achievable using SPM nanostructuring and nanomanipulation techniques. These techniques are inherently local and can hardly work in parallel, while with the new method the nanostructure would extend over the whole electrode surface, irrespective of its size, and several samples could be prepared simultaneously. We show here that copper can adsorb on a cyanide-modified * Corresponding author. E-mail: [email protected].

Figure 1. Ball model of the (23 × 23)R30° structure adopted by the cyanide adlayer on Pt(111). Blue balls correspond to Pt atoms, and orange balls correspond to irreversibly adsorbed CN groups.

Pt(111), exactly following, without disrupting it, the molecular pattern created by the cyanide adlayer, although the chemical state of the adsorbed Cu (either ionic or metallic) remains unknown to us. Cyanide adsorbs irreversibly on Pt(111) forming a (23 × 23)R30° structure7 consisting of hexagonally packed hexagons, each containing six CN groups adsorbed on top of a hexagon of Pt atoms surrounding a free Pt atom (Figure 1). It has been shown that H, OH, CO, or NO can adsorb on the uncovered platinum atoms within the troughs left free in the structure adopted by cyanide on the Pt(111) surface.8 Recently, we have found that methanol9 and formic acid10 can be oxidized on the free Pt atoms of a cyanide-modified Pt(111) electrode, which can therefore be used as a model surface to investigate the role of atomic ensembles in electrocatalysis. These results led us to think that cyanide-modified Pt(111) electrodes could be used to obtain a periodic metallic network formed by one-atom-wide lines extending over the whole electrode surface by electrodepositing a metal on the free Pt sites. In this first study we report on the adsorption of copper from the solution on a cyanidemodified Pt(111) electrode, yielding a one-atom-wide honeycomb superstructure. We chose Cu for the first experiments for three reasons: (i) its atomic diameter is small enough to fit in the troughs between the rings of CN groups, (ii) it deposits reversibly at underpotentials (underpotential deposition, upd) on Pt(111), thus allowing one to very easily stop the process just when one Cu atom has been deposited on every free Pt atom, and (iii) it forms very stable Cu(I) coordination complexes with cyanide. The use of a molecular pattern to design and control molecular structures at the nanoscale has already been reported,11 but our work presents two novelties: (i) the attempt

10.1021/jp901643q CCC: $40.75  2009 American Chemical Society Published on Web 06/12/2009

Atomic Scale Surface Decoration Using Molecular Pattern

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to use a molecular pattern to govern the deposition of a metal, and not the adsorption of an organic or organometallic molecule or supramolecule; and (ii) the line width of the pattern, of only one atomic diameter. Experimental Section The working electrodes used for cyclic voltammetry (CV) and Fourier transform infrared (FTIR) experiments were beadtype platinum single crystals (2 and 4 mm in diameter, respectively) prepared according to the method developed by Clavilier et al.,12 oriented and polished parallel to the (111) plane (miscut < 0.05°). The Pt(111) electrode used for STM experiments was a single crystal disk (10 mm in diameter) purchased from MaTeck (Ju¨lich, Germany). Before each experiment, the crystal was annealed in the flame of a Bunsen burner and cooled in a H2-N2 atmosphere. Cyanide-modified Pt(111) electrodes were prepared by immersion of a clean and well-ordered Pt(111) surface in a 0.1 M KCN (Merck, p.a.) solution for approximately 3 min, after which the electrode was rinsed with ultrapure water and transferred to the electrochemical cell containing the cyanide-free electrolyte (0.1 M H2SO4, or 0.1 M H2SO4 + 1 mM CuSO4 for the experiments involving copper deposition). The auxiliary electrode was a platinum wire. Either a reversible hydrogen electrode (RHE) (CV and FTIR experiments), or a platinum wire (STM experiments) were used as reference and quasi-reference electrodes, respectively. All the potentials in the text are referred to the RHE. A PicoLE Molecular Imaging STM with a PicoScan 2100 Controller was used. Experiments were performed with tungsten tips, etched from a polycrystalline wire in 2 M NaOH and coated with an electrophoretic paint in order to reduce the faradaic current at the tip/electrolyte interface. All images were recorded in the constant-current mode (IT ) 2 nA). In situ specular reflectance FTIR measurements were performed using a three-electrode spectroelectrochemical cell with a 60° CaF2 prismatic window attached to its bottom and placed in a Perkin-Elmer 1725X FT-IR spectrometer equipped with a narrow-band mercury-cadmium-telluride (MCT) detector, using p-polarized light. Each spectrum consisted of 100 interferograms, collected with a spectral resolution of 8 cm-1. The differential spectra were calculated as -log(Rsample/Rreference), where Rreference and Rsample are the reference and sample spectra, respectively, collected at different potentials. According to the equation above, positive absorption bands correspond to species present at the interface in the sample spectrum and absent in the reference spectrum, while negative absorption bands correspond to species present at the interface in the reference spectrum and absent in the sample spectrum. Bipolar bands would correspond to species present in both the sample and reference spectra, but whose frequency changed as a result of potential and/or coverage variations. Results and Discussion Figure 2 shows a cyclic voltammogram (CV) of a cyanidemodified Pt(111) electrode in cyanide-free 0.1 M H2SO4 + 1 mM CuSO4. The total charge obtained by integration of the CV between 0.80 V (where a small negative, continuously increasing, current starts to be observed; see inset in Figure 2) and 0.32 V amounts to -508 µC cm-2, indicating deposition of a full monolayer (ML). It should be noted that Cu upd (-480 µC cm-2 for a two-electrons process) is accompanied by desorption of cyanide (0.5 ML, -120 µC cm-2) and by adsorption of 0.22 ML of sulfate onto the Cu upd adlayer

Figure 2. First (red line) and second (black line) cycles at 1 mV s-1 of a cyanide-modified Pt(111) electrode. Blue line: CV at 1 mV s-1 of a Pt(111) electrode. The electrolyte was 0.1 M H2SO4 + 1 mM CuSO4 in all cases. The inset shows the first negative sweep in an expanded scale.

(+105.6 µC cm-2 for 0.22 ML onto 1 ML of Cu on Pt(111)), as suggested by the fact that the subsequent positive sweep is nearly identical to that of unmodified Pt(111) in 0.1 M H2SO4 + 1 mM CuSO4 (compare the black and blue positive sweeps in the CVs in Figure 2), but for a slightly smaller charge under the peak. In the subsequent negative sweep, Cu upd occurs at the same potential as with unmodified Pt(111) in 0.1 M H2SO4 + 1 mM CuSO4 (compare the black and blue negative sweeps in the CVs in Figure 2), although the charge is smaller and the most positive peak has nearly disappeared. This most positive peak of the Cu upd process has been attributed to the formation of a (3 × 3)R30° structure consisting of both Cu and (bi)sulfate ions,13 and we suggest that the absence of this peak in the second negative sweep (black line in Figure 2) is due to the presence of a small amount of CN groups hindering the formation of large domains of the (3 × 3)R30° structure. These results are in agreement with an earlier study by White and Abrun˜a,14 who studied Cu upd on Pt(111) electrodes pretreated with, or in the presence of, several anions and molecules, among them cyanide. However, we must note that, since their study was restricted to voltammetric measurements, they could not detect the presence on the surface, at potentials between 0.58 and 0.80 V, of copper adsorbed on the cyanidemodified Pt(111) electrode, exactly following the pattern created by the CN groups (see below). Cyanide-modified Pt(111) electrodes in Cu-containing solutions were studied by electrochemical STM (EC-STM). In the region between 0.80 and 0.58 V, where a small, negative, continuously increasing current can be observed (see inset in Figure 2), a honeycomb structure with one-atom-wide walls (Figure 3) is observed. This structure was not observed in Cufree solutions, and was therefore assigned to copper adsorbed on the cyanide-modified Pt(111) electrode. The distance between the centers of two adjacent hexagons is ca. 9.5 Å, corresponding to the distance expected if the copper is adsorbed on the Pt atoms surrounding the CN rings (23dPt ) 23 × 2.77 Å ) 9.6 Å) of the cyanide-modified Pt(111) electrode (see Figure 1). Accordingly, the distances between two opposite corners (4dPt) and two opposite sides (23dPt) are ca. 11.8 and 9.5 Å, respectively. Assuming that one Cu ion or atom adsorbs per free Pt atom around a CN ring, the total Cu coverage corresponding to the honeycomb structure observed by STM amounts to 0.42 mL.

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Figure 3. (a) STM image (26 × 25 nm2) of a cyanide-modified Pt(111) electrode in 0.1 M H2SO4 + 1 mM CuSO4 at E ) 0.68 V. (b) Zoom (12 × 11 nm2) on the central terrace showing the honeycomb Cu structure. UT ) 0.38 V (tip negative).

Escudero et al.

Figure 5. STM image (50 × 50 nm2) of the cyanide-modified Pt(111) electrode in 0.1 M H2SO4 + 1 mM CuSO4 at E ) 0.52 V. UT ) 0.22 V (tip negative).

Figure 6. (A) STM image of the cyanide-modified Pt(111) electrode in 0.1 M H2SO4 + 1 mM CuSO4 at E ) 0.35 V, showing a 46 × 46 nm2 area covered by a Cu upd adlayer, onto which the well-known (3 × 7)R30° structure of adsorbed sulfate has formed. UT ) 0.05 V (tip negative). The inset shows a molecularly resolved image of the same surface (24 × 12 nm2). (B) STM image (200 × 200 nm2) recorded during a positive potential sweep from 0.35 to 0.85 V. The scan direction is from top to bottom and from left to right. The dissolution of the Cu upd adlayer can be observed in the upper third of the image. UT ) 0.05 to 0.50 V (tip negative).

Figure 4. STM image of the cyanide-modified Pt(111) electrode in 0.1 M H2SO4 + 1 mM CuSO4 at E ) 0.68 V, showing a 30 × 30 nm2 area covered by the honeycomb Cu structure. UT ) 0.34 V (tip negative).

The pattern formed extends over the whole electrode surface (Figure 4), although it shows defects. At this point, we do not know whether these defects are intrinsic to the copper superstructure or whether they correspond to defects within the cyanide template. When the potential is further decreased below the shoulder at 0.58 V in the corresponding CV (see inset in Figure 2), the honeycomb structure coexists with islands onto which a different structure can be observed (Figure 5). Although we could not resolve the latter structure clearly, the observation on some of the islands of three rotational domains, as expected for the (3 × 7)R30° sulfate structure, suggests that they correspond to epitaxial copper upd islands on the Pt surface.

As we reach the potential of the peak at 0.37 V in the first negative sweep in the CV (Figure 2, red line and inset), a nearly complete copper upd monolayer is formed concomitantly with the desorption of the cyanide adlayer and the adsorption of sulfate onto the Cu upd adlayer, as indicated by the observation over the whole electrode surface of the well-known (3 × 7)R30° structure, characteristic of sulfate adsorbed on the (111) faces of most face-centered cubic (fcc) metals15 (Figure 6A). This Cu upd adlayer is stripped in the anodic peaks in the positive sweeps of the CV (Figure 2 and Figure 6B). Recently, Hsieh and Gewirth16 have shown that linear oneatom wide arrays of Pb can be formed by interaction of Pb upd adlayers with thiocyanate, but, contrary to our work, thiocyanate was not preadsorbed, but present in the solution, and deposition of lead was not forced to follow a preformed pattern. In situ FTIR spectra recorded at potentials between 1.00 and 0.30 V with a cyanide-modified Pt(111) immersed in a 0.1 M H2SO4 + 1 mM CuSO4 solution confirm the conclusions reached

Atomic Scale Surface Decoration Using Molecular Pattern

Figure 7. (a) Potential difference-FTIR spectra of a cyanide-modified Pt(111) electrode in 0.1 M H2SO4 (left), and in 0.1 M H2SO4 + 1 mM CuSO4 (right). The reference spectrum was recorded at 1.30 V. (b) Plot of the integrated absorbance of the cyanide band as a function of the electrode potential in 0.1 M H2SO4 (circles) and in 0.1 M H2SO4 + 1 Mm CuSO4 (squares).

from the EC-STM data. At E > 0.60 V, bands corresponding to carbon-bonded, linearly adsorbed CN can be observed, in the same frequency region and with a similar intensity as that observed in the absence of Cu(II) in the solution (Figure 7a), confirming that the cyanide adlayer remains essentially intact in the potential region where the honeycomb Cu structure is observed (Figure 3). No other band indicating the formation of another carbon- or nitrogen-containing species during the experiment could be observed in the spectra. At E e 0.60 V, the intensity of the CN stretching band decreases as compared with that observed in the same potential region in the absence of Cu(II) (Figure 7a,b), and a band around 1220 cm-1, corresponding to adsorbed sulfate, starts to appear (Figure 8), confirming the formation of a sulfate-covered Cu upd adlayer on Pt(111) in this potential region, and the destruction of the CN pattern, as deduced from the CV (Figure 2) and the ECSTM images (Figure 5). Obviously, the knowledge of the oxidation state of copper in the honeycomb structure observed in the potential region between 0.80 and 0.58 V is crucial for possible applications of our method. We performed ex situ X-ray photoelectron spectra (XPS), after emersion of the cyanide-modified Pt(111) electrode from a cyanide-free 0.1 M H2SO4 + 1 mM CuSO4 at 0.68 V and transfer to an ultrahigh vacuum (UHV) chamber through the laboratory’s and the patio’s atmosphere. Although these kinds of ex situ experiments have to be taken with great care, since changes in the chemical nature of the species present on the electrode surface can occur as a result of the loss of potential control upon emersion and transfer to the UHV chamber, we expected XPS to confirm the presence of copper adsorbed on the surface in the potential region in which the honeycomb structure is observed by EC-STM. According to XPS, Cu(I) is present on the electrode surface (see Supporting Information, Figures S1 and S2, and Table S1), suggesting that the structure observed in situ by STM must correspond either to Cu(I)

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Figure 8. Potential Difference-FTIR spectra of a cyanide-modified Pt(111) electrode in 0.1 M H2SO4 + 1 Mm CuSO4. The reference spectrum was recorded at 1.00 V.

stabilized on the surface by the CN groups, or to metallic copper which was oxidized to Cu(I) during the transfer of the sample to the UHV chamber. A third possibility is that Cu initially present as Cu(II) on the electrode surface was reduced to Cu(I) upon irradiation with X-rays. Changes in the chemical nature of the adsorbed species upon emersion of the electrode from the solution and transfer to the UHV chamber are evident from the N 1s XPS spectrum (see Supporting Information, Figure S3, and Table S1): although only one component, namely, that corresponding to carbon-bonded, linearly adsorbed CN, was expected, several were found, including some at 400.5 and 401.7 eV attributable to other N chemical species different from CN. Similarly, the O 1s XPS spectrum shows a high binding energy contribution, which might be attributable to C-O bonds (see Supporting Information, Figure S4, and Table S1). Taking into account that, in addition to carbon-bonded, linearly adsorbed CN, no other nitrogen-containing species was observed in the in situ FTIR spectra recorded in the potential region where the honeycomb structure is stable, it must be concluded that the multiplicity of nitrogen-containing species observed by XPS after emersion of the electrode from the electrolyte must be due to uncontrolled chemical reactions occurring upon exposure of the sample to the laboratory’s atmosphere or during the XPS measurements. It must be noted that the charge flowing through the interface in the region between 0.80 and 0.58 V, where a small negative, continuously increasing, current can be observed (see inset in Figure 2), is too small (41 µC cm-2) to account for the formation of 0.42 mL (the coverage corresponding to the honeycomb structure) of either Cu(I) (101 µC cm-2) or metallic Cu (202 µC cm-2) from Cu(II). However, the occurrence of parallel processes, such as sulfate coadsorption, could counterbalance the charge due to reduction of Cu(II), making the charge analysis far from straightforward.

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Conclusions We have shown that a periodic network of atomic dimensions, perfectly following a pre-existing molecular pattern, can be formed by adsorption of copper on a Pt(111) electrode chemically modified by cyanide, which acts as a template that allows decoration of the electrode surface at the atomic scale. Our goal is to fabricate periodic metallic nanostructured deposits by using a molecular template, but at this point it remains unknown to us if, in the potential region where the honeycomb structure can be observed (between 0.80 and 0.58 V), copper adsorbs on the cyanide-modified Pt(111) surface in a metallic or an ionic state. Surface X-ray scattering experiments and theoretical calculations, which hopefully will help us clarify this very important point, are currently in progress. Acknowledgment. Funding from the DGI (Spanish Ministry of Education and Science) through Projects CTQ2006-02109 and CTQ2006-26289-E is gratefully acknowledged. M.E. acknowledges an FPI fellowship from the Spanish Ministry of Education and Science. We thank Prof. Claudio Gutie´rrez for a critical reading of the manuscript. Supporting Information Available: XP spectra, with the corresponding experimental details and discussion. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Binnig, G.; Rohrer, H.; Gerber, Ch. Phys. ReV. Lett. 1982, 49, 57– 61. (2) (a) Becker, R. S.; Golovchenko, J. A.; Schwarzentruber, B. S. Nature 1987, 325, 419–421. (b) Eigler, D. M.; Schweizer, E. K. Nature 1990, 344, 524–526. (c) Crommie, M. F.; Lutz, C. P.; Eigler, D. M. Science 1993, 262, 218–220. (3) Walter, E. C.; Murray, B. J.; Favier, F.; Kaltenpoth, G.; Grunze, M.; Penner, R. M. J. Phys. Chem. B 2002, 106, 11407–11411.

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