Electrode surface modified with heteropoly or isopoly anions: STM and AFM characterization

It has been shown on two examples that STM and AFM techniques can be used to image oxometalates with molecular resolution. The shapes of these molecules are clearly distinguished and their dimensions have been determined. It has even been possible to correlate other results culled from the AFM image of the surface of a single crystal of an oxometalate with those of crystallography. These results served as reference images in the study of the basal plane of HOPG when it is used as a working electrode during the cyclic voltammetry of 10-3 M H ~ w ~ ~ $ , in a pH = 2 medium, with simultaneous STM monitoring of the surface morphology. A potential-dependent deposition of the oxometalate is superimposed on a strong alteration of the HOPG surface. When the potential domain is extended to include the third redox system of H ~ w ~ ~ C ~ & , a particularly well-ordered two-dimensional array is observed, the individual entities having the diameter of the oxometalate. Recently, the catalytic properties of electrode surfaces modified with heteropoly or isopolyanions have been described in several papers from this laboratory [l-41. Two types of modifications were studied. Materials as diverse as glassy carbon, graphite, gold, tungsten, etc, or semiconductor like p-type Si have been modified by direct electrodeposition of oxometalate-based catalysts. Also, it has been possible to include these oxometalates in organic polymers or in clays, previously deposited on electrode surfaces. Whatever the technique, the observed catalytic properties must be undoubtedly by traced to the presence of the oxometalates or to their reduction products. Therefore, these species which appear as molecular soluble analogs of metal oxides or metal oxide mixtures, offer a rare opportunity to study chemical and structural transformations of the catalysts on well-defined species. As a matter of fact, little is known about the geometrical factors within the molecules, or their spatial distribution or chemical transformations which, all together, could induce the observed catalytic properties. Then, nearfield microscopies, many of which can be operated in-situ during electrochemical experiments, appear as very appropriate tools to try and monitor topographical changes related to volume properties in catalysts at work. Among the necessary steps towards this end, it must be proved that heteropoly and isopolyanions can be imaged with a high spatial resolution. The first part of this paper is therefore devoted to the STM and AFM study of this class of compounds [S-81. The second part discusses some results obtained when electrode surfaces are imaged in-situ at a molecular or even an atomic resolution during the scanning of the cyclic voltammogram of an oxometalate [9]. A Nanoscope 11, operating in air and at room temperature was used throughout (Digital Instruments, Santa Barbara, CA). Properly implemented, the same apparatus is used for AFM experiments. The Nanoscope I1 can also be equipped with a bipotentiostat for simultaneous electrochemical measurements and STM or AFM imaging (ECSTM or ECAFM). For ECSTM, W or P a r (80/20) tips are coated with Apiezon was, except for their very end. To further minimize the faradic current through the tunneling tip, the tiplsurface bias voltage was usually fixed so that parallel variations occur in the tip/electrode potential during voltammetric runs. Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jp4:1994126 C1-330 JOURNAL DE PHYSIQUE IV Ex-situ STM and AFM studies [5,6, 81 P W ~ ~ O ~ ~ which is one of the precursors used for electrode modications 11-41 has been selected as a first example. Like most transition metal polyoxoanions, it is constituted by discrete molecules, with their structures based on close-packed oxygen arrays containing interstitial metal centers. This very symmetrical structure is easily accommodated in a sphere of 11A diameter. A drop of methanolic solution of H3PW12040 (typically 10-4M) was deposited on the freshly-cleaved surface of a piece of highly oriented pyrolytic graphite (HOPG) and the solvent was allowed to evaporate. Figure 1 shows current-mode STM images of this surface. The observed structure is stable, well-ordered in several areas, with parameters clearly different from those of the underlying HOPG. In the constant currentmode STM, the tunnel tip traces out contours of constant density of states and thus, probes the topology of the electron distribution above the surface of the sample. Therefore, the structures in figure 1, which closely resemble what could be expected from the crystallographic shape of P W ~ ~ O & , suggest that individual oxometalate species are imaged. The mean diameter, deduced from numerous measurements [5] is 9.6 i 0.4 A, which is very close to the crystallographic diameter. With the aim of further demonstrating the ability of STM and AFM techniques to show the shape of oxometalates and to help in their size determinations,~~diumdecatungstocerate(IV)wasselected as a new representative example. This molecule Na&12CeW10036 30 H20 is rather ellipsofdal in shape. STM studies on this compound deposited on HOPG and AFM imaging on a crystal wet with solvent give very similar results, from which the molecular dimensions could be measured. Other conclusions could be drawn as well [6, 81 : i) in the case of oxometalates, a substrate like HOPG has no detectable influence on the STM image. ii) the STM image corresponds to a purely topographic representation of the surface. iii) for oxometalates, the correlation between images obtained by the two techniques is excellent. These conclusions have received confirmation through the AFM and crystallographic studies of a single crystal of sodium decatungstocerate (IV). Figure 2 shows the AFM image of the surface of such a "dry" crystal. Even though slightly "faulted" areas exist, the pattern is largely dominated by a periodic arrangement of molecules. The molecular shape is easily observed, and the molecular dimensions have been measured with a fair accuracy. The very regularity of the observed pattern prompted us to try and fit the AFM images with the crystallographic arrangements of the molecules in the single crystal. Sodium decatungstocerate (IV) crystallizes with 30 water molecules and belongs to the monoclinic system, space group C2/c, a = 18.14 A, b = 18.62 A, c = 18.51 A, P = 95.9' and Z = 4. We have generated a computer representation of the arrangement of the heteropolyanion in a projection along the b-axis. This arrangement has been chosen because it seems to correspond best to the AFM image. The fitting of this computer arrangement to a zoomed AFM pattern shows that the angles in the apparent unit-cell are slightly different from 90'. Fig. I . STM images of H,PW,,O, in air at 298 K ( I = 1.1 nA. U = 201.4 mV). Images have been filtered by a two-dimcnsional fast-Fourier-transform. Pitch anglc: 309. (a) Three dimensional line-plot image. (b) Zoomed thrcc dimensional lineplot imagc. Also, the experimental distances deduced from the AFM image appeartoapproachcrystallographic determinations to within less than 5 %. All these results together suggest that the scanned area represents actually the (010) face of the crystal and appears as a simple termination of the bulk. Simultaneous STM imaging and cyclic voltammeby [9] The conclusion from the aforementioned results is that fairly adherent deposits of the oxometalates on HOPG can be imaged by STM or AFM, showing recognizably the shapes of the molecules and allowing for determination of molecular dimensions. Starting with such reference images, an electrochemical study with simultaneous STM imaging has been undertaken with a solution containing 1 e 3 M (NH4)6 H2W12040, in a pH = 2 medium. The electrochemical cell was the one received with the Nanoscope, with two Pt wires as counter and reference electrodes respectively. The working electrode is the basal plane of a freshly etched HOPG of 0.41 cm2 surface area. It has been checked that the Pt wire acts as a quasi-reference electrode : as a matter of fact, the present oxoanion is known to generate stable reduction products upon its first several electronations, and to constitute nernstian systems with them. This assumption is reinforced by the following observation : the cathodic 6peak potential difference between the first two reduction waves of H2W1204,,, measured with the present electrochemical set-up, is and remains to less than 10 mV the same as obtained in a conventiona1 set-up with a saturated calomel electrode as the reference. Figure 3 shows a reference 6cyclic voltammogram of H 2 W 1 2 0 ~ ~ in a pH = 2 medium, restricted to the fust three waves and runned in a conventional electrochemical cell. The first two waves are bielectronic, the third one corresponds to approximateIy ten electrons [lo, 111. All the other cyclic voltammograms have been obtained in the ECSTM cell. 10 Figure 2 : Line-plot AFM image of the surface of sodium decatungstocerate (IV) in air at room temperature. The image has been filtered by twodimensional fast Fourier-nansform. 5