J. MATER. CHEM., 1991,1(4), 517-523 Scanning Tunnelling Microscopy Study of Osmium-containing Electroactive Metallopolymer [Os(bipy),(PVP),,CI]CI Films on Polycrystalline Graphite Electrodes Norman M. D. Brown,*a Hong Xing You/ Robert J. Forsterband Johannes G. Vosb a Surface Science Laboratorx Department of Applied Physical Sciences, University of Ulster at Coleraine, Co. Londonderrx BT52 ISA, Northern Ireland, UK School of Chemical Sciences, Dublin City Universitx Dublin 9, Ireland Scanning tunnelling microscopy (STM) has been applied to the study, in air, of the topography of Os(bipy),CI,- loaded poly(4-vinylpyridine) (PVP) (bipy=2,2'-bipyridyl) films supported on polycrystalline graphite. Two markedly different topographical features are observed on the sample surfaces before and after electrochemical cycling in aqueous sulphuric acid electrolytes.These features are characterized, respectively, by ordered domains with rows of fibrillar structure and ordered domains with rows of granular structure. Electrochemical cycling in the sulphuric acid electrolyte therefore clearly has a profound effect on the topographies of the electroactive polymer fi Ims. Keywords: Scanning tunnelling microscopy; Electrochemical cycling; Modified electrode; Electroactive polymer; Topography 1. Introduction Metallopolymer-modified electrodes have shown potential applications in electrocatalysis,' photoelectrochemistry2 and macromolecular electronic^.^ The useful applicability of the metallopolymer-modified electrodes in many of these areas will be determined by the mechanism of charge transport through the polymer film.This electrical charge transport has not only a strong bearing on the proper design of the polymer microstructure but also on the topography of the polymer surfaces concerned. Thus, the transverse motion of a charge carrier can be prevented by the physical intervention, for example, of topographical wells: i.e. a local surface pit. In order to explain the electronic properties of these polymer- modified electrodes fully, it is essential to have a clear under- standing of the structures of the conducting modifying polymers. For particularly regular polymer structures, X-ray or elec-tron diffraction can be used to analyse both the relative position of the atoms in the molecular repeat units, i.e.the unit cell, and the arrangement of the unit cell along the molecular chains. Spectroscopic techniques, such as infrared spectroscopy (IR), nuclear magnetic resonance spectroscopy (NMR) and X-ray photoelectron spectroscopy (XPS), can also be used collectively to probe the microstructure of the polymer molecules and can give information on the conformation and packing of the polymer chains involved and the chemical states of the atoms or ions present. However, these techniques do not provide sufficient information to determine in detail the structure of the polymer matrix ~oncerned.~ The general topographies of polymers can be viewed comp- lementarily with an optical or an electron microscope at low or moderately high resolution, i.e.from micrometres to several thousand Angstroms, respectively. On the other hand, some other electron and ion microprobe techniques, for instance low-energy electron diffraction (LEED), Auger electron spec- troscopy (AES) or scanning Auger microscopy (SAM), employing the high-energy electron beams to excite Auger emission of electrons, may cause radiation damage to the polymer surfaces of interest.' This problem has restricted, to some degree, the application of the above microprobe tech- niques in the studies of polymer surfaces. Thus, resort is made to novel or other complementary techniques for the structure determination of polymers. Scanning tunnelling microscopy (STM), since its emergence in 1982,6 has proved to be a powerful tool in surface science, uniquely in the determination of surface topography at ultra- high resolution, i.e. in scales from nanometres to a few Angstroms, whether in vacuum, air, or liquid environments.At the outset, scanning tunnelling microscopy was applied mainly to metal and semiconductor surfaces. Recently, how- ever, scanning tunnelling microscopy has been turned directly to the study of polymers and other organic With the ultra-low electron energy used (a few eV at most) and without mechanical contact, the STM technique is non-destructive and comparatively flexible. Hence, it appears to be ideal for the surface study of polymers or organic films in certain circumstances.The main limitations of the STM technique lie essentially in the need for the sample to be electrically conductive and the sample surface to be reasonably flat. In general, polymers and other organic films are poor electrical conductors. Therefore, they are excluded from STM studies if they are not conductive intrinsically and sufficiently or if a preliminary surface metallization is not performed. Vacuum deposition of thin layers (<I0 nm) of conductive material (Pt/C or Pt/Ir/C) on the polymer surface or the organic film~~*"7'~ can solve, to a certain degree, this conduc- tivity problem with a loss, because of the overlayer, in the ultimate spatial resolution attainable. Given these restrictions, only a few STM studies have so far been done on conductive polymers per se,l1-I4 and on the polymer films deposited on graphite and gold substrates by Langmuir-Blodgett (LB) technique^.'^-'* In this paper, scanning tunnelling microscopy is applied to the study, in air under ambient conditions, of the osmium- containing metallopolymer films { [Os(bipy),(PVP) oC1] C1) coated on polycrystalline graphite electrodes.The topograph- ical images, obtained from the samples before and after the electrochemical cycling in a sulphuric acid electrolyte solution, show two striking features. These are, first, ordered domains with the unidirectional rows of fibrillar structure, typically on the sample surfaces prior to the electrochemical cycling and, secondly, ordered domains with aligned rows of granular structure, particularly for samples following the aqueous sulphuric acid cycling.The experimental results presented amply demonstrate the applicability of scanning tunnelling microscopy to the topographical study of the surfaces of the conducting polymer-modified electrodes, revealing the micro- structure of the polymer overlayer at ultra-high resolution. Such methods can clearly be extended to the working electrode in an aqueous environment. 2. Experimental 2.1 Sample Preparation The polycrystalline graphite electrodes of 7 mm diameter mounted in Teflon shrouds, used in the experiments for the polymer overlayer growth, were cut sections of polycrystal- line graphite rod (Agar Aids Ltd., Essex) prepared by mechan- ical polishing using a 0.5 pm alumina slurry on a felt pad followed by thorough washing with distilled water and methanol.The poly(4-vinylpyridine) (PVP) was prepared from freshly distilled 4-vinylpyridine by bulk polymer-isation under nitrogen at 70-75 "C using 2,2'-azoisobutyro- nitrile as initiator. The crude polymer was purified there- after by repeated precipitation in diethyl ether from methanol. The molar mass, as determined by viscometry in absolute ethanol, was found to be 4.3 x lo5 g. The Os(bipy),Cl, complex was prepared by standard laboratory procedures. The modifying metallopolymer was prepared, as reported for the corresponding ruthenium-containing polymer, by refluxing Os(bipy),Cl, with a 10-fold excess of PVP in ethanol.Finally, the adherent electrode coatings were ob- tained by evaporating a few mm3 of a 1% solution of the osmium-containing modifying polymer in ethanol onto the electrode surface in a solvent-vapour saturated chamber followed by air drying to give a film ca. 4000 A thick. The detailed sample preparations and the electrochemical pro- cedures and equipment are described elsewhere." The mol- ecular structure of the thus prepared osmium-containing metallopolymer ([0~(bipy)~(PVP),,Cl]Cl}is shown in Fig. 1. In the study of electron transport in the osmium-containing metallopolymer or any redox polymer,' using chronocoulome- try or cyclic voltammetry, the redox polymer-modified elec- trode will normally undergo electrochemical cycling.In use therefore, the coated electrode has its potential controlled on one side and is wetted on the other by the solvent of the electrochemical cell and any supporting electrolyte. Sulphuric acid (H2S04)solutions with concentrations in the range 0.2- L N J Fig. 1 Molecular structure of the osmium-containing metallopolymer [Os(bipy),(PVP),,Cl]CI (N-N =2,2'-bipyridyl) J. MATER. CHEM., 1991, VOL. 1 1.0 mol dm-3 were chosen in this case as the supporting electrolyte. The electrochemical cells were of conventional design and were thermostatted to &1 "C.19 2.2 STM Instrumentation The scanning tunnelling microscope used (W. A. Technology Ltd., Cambridge) consists of a scanning head suitable for operation in either air or vacuum, an electronic control unit, a Tandon 386 microcomputer with VGA colour monitor and frame store facility, and a monochrome monitor for image display. The tip used is produced from a 0.5 mm diameter tungsten wire (99.99%) which was etched, by a routine electro- chemical method, in an aqueous 1 mol dmP3 KOH solution.20 A more detailed description of this STM instrument can be found elsewhere.20'2' The STM images presented were obtained in the constant tunnel current mode, from the osmium-containing metallopolymer-modified polycrystalline graphite electrodes before and after electrochemical cycling in sulphuric acid electrolytes of varying concentrations.The bias voltages were in the range 700-1600 mV, with the tunnel current set between 0.6 and 1.2 nA.The typical scanning speeds used in the X or Y direction scans were between 0.1 and 5 nms-'. A typical sample surface, on inspection through an optical microscope, generally looked very smooth and shiny before the electrochemical cycling, whereas it was flat and grey in appearance after cycling. Here we present the STM results for samples, treated in sulphuric acid electrolyte solutions of 0.2 and 0.4 mol dm-3, reproducible STM images could be obtained over a considerable area. For the samples exposed to the sulphuric acid electrolytes at higher concentrations, the surfaces became less dense and visibly swollen. The STM images obtained from these were unsatisfactory because of the swollen nature of the surfaces and are not considered further.3. Results 3.1 Scanning Tunnelling Microscopy The STM images obtained from osmium-containing polymer- modified polycrystalline graphite electrodes before electro- chemical cycling are shown in Fig. 2. These images were recorded at a bias voltage of 900 mV (with the sample positive) and at a tunnel current of 0.8 nA. Fig. 2(a), typical of a larger area (here scanned over 800 Ax800 A), illustrates some straight fibrils running through the scanned area with the same general orientation. Moreover, it can also be seen from Fig. 2(a) that there are other branch-like fibrils running in a zigzag manner beside the straight fibrils. Some of these branch- like fibrils extend one end in another direction and then line up parallel with the straight fibrils.Some extend close to the sides of the straight fibrils and finally join them. The smaller- scale scan expanded from the left part of the straight fibrils in Fig. 2(a) is shown in Fig. 2(b). As can be seen clearly, there are several straight fibrils on the surface with a side-by-side distance or a fibrillar width of ca. 20 A. Fig. 2(c) shows another frequently observed feature, i.e. a series of steps, on the sample surfaces before electrochemical cycling.The steps exhibit rough and uneven edges with an edge-to-edge distance of ca. 20 8, and with step heights in the range 8-9 8,. The STM images of the samples, after being exposed to the 0.4 mol dm-3 sulphuric acid electrolyte, shown in Fig. 3 and 4, are typical of those observed for such samples.These images were recorded at a bias voltage of 1400 and 1350 mV, respect- ively, and at a tunnel current of 1.0 nA, over the scanned areas of 800 8, x 800 8, and 1000 8, x 1000 A. As shown, J. MATER. CHEM., 1991, VOL. 1 Fig. 2 STM images of an osmium-containing metallopolymer-modi- fied polycrystalline graphite electrode before electrochemical cycling in the sulphuric acid electrolyte, recorded at a bias voltage of 900 mV and a tunnel current of 0.8 nA. (a) A large-scale image (image size: 800 A x 800 A x 68 A); (b)the smaller-scale scan expanded from the left side of (a)showing rows of the rod-like structure (image size: 276 8, x 276 A x 54 A); (c) a series of the steps (image size: 106 A x 106 Ax16 A) Fig.3(a)and 3(b)are actually the same image, but to emphasize the featured area which is rather diffuse in Fig. 3(a), image contrast enhancement processing was used for Fig. 3(b). In this, the image intensity data values of each pixel are adjusted up or down so that, in the displayed image, there is an equal number of pixels of each intensity. A smaller scale scan zoomed from the featured area in Fig. 3(a)is shown in Fig. 3(c). It can be seen clearly that the featured area in Fig. 3(a)consists of rows with a granular structure which sit side-by-side aligned in one direction. The distance between two such granules from centre to centre was, typically, ca. 40 A, with the granules showing considerable size uniformity, i.e. ca. 40 8, x 40 A or ca.40 8, x 30 A. The side-by-side distance of the rows apparent in Fig. 3(c) was found to be, typically, ca. 40 A, the same as the granular centre-to-centre distance visible in the longitudi- Fig. 3 STM images of an osmium-containing metallopolymer-modi- fied polycrystalline graphite electrode after exposure to a 0.4 mol dm- aqueous sulphuric acid electrolyte, recorded at a bias voltage of 1400 mV and a tunnel current of 1.0 nA. (a)A large-scale image showing rows of the granular structure (image size: 800 8,x 800 Ax81 A); (b) the same image as (a) but with the image contrast enhanced; (c) the smaller-scale scan ex anded from the featured area of(a) (image size: 313 A x 313 AX 24 1) nal direction. Fig. 4 shows the overlapping layer structure (or wide fibril") frequently observed on the sample surfaces having been exposed to the 0.4 mol dmP3 sulphuric acid electrolyte.Fig. 4(b) is a high magnification image of the middle of Fig.4(a), showing the fine step structure. As seen in Fig. 4(b), the steps have relatively smooth and even edges. These edges lie approximately parallel to the diagonal of the image, i.e. at an angle of ca. 135" with respect to the X axis. The step heights found vary from ca. 30 A for the first and second steps (counting from the upper right-hand corner), ca. 40 A for the third and ca. 60 A for the fourth. Correspond- ingly, the respective edge-to-edge distances of the steps have different values as well, i.e. ca. 60, ca. 90 and ca. 120 A.In these experiments, the step height and the step edge-to-edge distances were found to change with the specific local tip Fig. 4 STM images of an osmium-containing metallopolymer-modi- fied polycrystalline graphite electrode after exposure to a 0.4 mol dm-3 aqueous sulphuric acid electrolyte, recorded at a bias voltage of 1350 mV and a tunnel current of 1.0nA. (a) A large-scale image showing the overlapping layer structure (image size: 1000 Ax1000 Ax236 A); (b) the high magnification image of (a) taken from the middle showing steps (image size: 423 A x 416 A x 151 A) location on the sample surface, but the corresponding param- eters of the similar features identified were frequently close to the above values or within the range of the values given.Occasionally, but consistently, the step heights found were only half of the above step heights. Typical STM images of samples examined after exposure to the 0.2 mol dmP3 sulphuric acid electrolyte are shown in Fig. 5. These images were recorded at a bias voltage of 1200 mV and at a tunnel current of 1.0 nA. From Fig. 5(a), two distinct features can be seen, i.e. the granular structure of the left part of the image and the overlapping layer structure of the remainder of the image. To show these topographical features more clearly, two high magnification images from the left and the right halves of Fig. 5(a) are shown in Fig. 5(b) and 5(c), respectively. In Fig. 5(b) it can be seen that the granular structure runs roughly parallel in the same general direction, with typical granule sizes of ca.80 81 x 80 81 or ca. 70 8, x 6081, respectively. The centre-to-centre distances of adjacent granules are, typically, ca. 90 81 both laterally and longitudinally. In Fig. 5(c), there are marked steps lying approximately parallel to the diagonal of the image, i.e. at an angle of ca. 135", as before, with respect to the X axis. The step heights are measured to be ca. 20 or ca. 30 A,respectively, with an edge-to-edge distance of ca. 80 8,. In these experiments, this mixture of granular structure and overlapping layer structure was frequently observed on the surfaces which had been treated in the 0.2mol dm- sulphuric acid electrolyte. Some of the structured regions on the surfaces were wholly composed of granular structured domains, and by contrast, others were totally composed of domains showing the overlap- ping layer structure.These two types of differently structured J. MATER. CHEM., 1991, VOL. 1 Fig. 5 STM images of an osmium-containing metallopolymer-modi- fied polycrystalline graphite electrode after exposure to a 0.2 mol dmP3 aqueous sulphuric acid electrolyte, recorded at a bias voltage of 1200 mV and a tunnel current of 1.0 nA. (a) A large-scale image (image size: 700 8,x 700 8, x 96 A); (b)the high-magnification image taken from the left half of (a) showing the granular structure (image size: 301 A x 301 8, x 74 A); (c)the high-magnification image taken from the right half of (a) showing steps (image size: 301 A x 340 A x 66 A) regions in some locations on the sample surface had a clear demarcation line while in other locations this line was not so obvious.For example, some of the granules lay side-by-side individually, while others overlapped each other and formed straight rows or wide fibrils. 3.2 Electrochemistry A cyclic voltammogram obtained for [0~(bipy)~(PVP)~~Cl]Cl in 0.4 mol dm-3 H2S04 is shown in Fig. 6. Upon varying the electrolyte concentration the same behaviour for the homo- geneous charge-transfer process within the complex-loaded layer was observed, as reported in an earlier paper22 in which J. MATER. CHEM., 1991, VOL. 1 li Fig. 6 A cyclic voltammogram of a polycrystalline graphite electrode coated with the [Os(bipy),(PVP )loCl]Cl polymer films.Electro-lyte 0.4 mol dm-3 HJO,, scan rate 100 mV s-', coverage 3 x lop8mol ~m-~,estimated layer thickness 4300 a detailed discussion of the electrochemistry of these modified electrodes in H2S04 and in other solvents is given. However, it should be emphasized here that the charge transport and kinetic parameters obtained for the osmium- containing metallopolymer show a large dependence on the nature and concentration of the electrolyte.22 The activation parameters obtained suggest strongly that at high sulphuric acid concentrations a substantial amount of swelling of the polymer matrix occurs. Cyclic voltammetry experiments sug- gest that in 1.0 mol dm-3 H2S04 polymer movement is rate determining; at lower electrolyte concentrations the process is controlled by ion movement. This suggests that at higher electrolyte concentrations the distance between the redox active osmium sites increases.The STM data elaborated here are believed to be consistent with these electrochemical obser- vations. After electrochemical cycling the surface structures of the layers have clearly been altered. These changes become more pronounced with treatments involving the increase of electrolyte concentrations. 4. Discussion Although artefacts can be observed in the STM imaging of organic films because of the attendant interaction of a flexible or poorly conducting organic film with the STM tip,7 the observed topographies described here are believed to be real for the following reasons.First, the features identified on the osmium-containing metallopolymer-modified polycrystalline graphite electrodes are consistently imaged in a large number of locations on a given sample surface and from a succession of samples, irrespective of the scan directions, scan speeds, bias voltage (within reasonable limits) or bias directions (Note, 521 there are some sample surface positions where the local conductivity is too poor for adequate STM image quality). Secondly, the row-like structures lie at various different angles, since a given sample has structured domains with different local alignments, and thirdly, the topographical features pre- sented here do not arise from the polycrystalline graphite substrate used because the known thickness of the polymer films coated on the carbon substrates, as indicated earlier, is ca.4000 A.Moreover, STM images of the uncoated polycrys- talline graphite substrate do not show any topographical features of similar appearance over the same range of scanned area scales. As a result, the topographical contributions of the substrate itself need not be considered further. The row-like structures of fibrils and granules seem to be the common topographical features observed on the surfaces of the sample electrodes studied. Similar structures have also been observed on the other polymer Therefore, such structures may be a consequence of local polymer ordering or ~ystallization'~*'~-'~ arising in the films, reflecting in turn the allowed molecular pa~king.'~,'~,'~ Now, in the imaging experiments described here, the ordered structures could be traced over several hundred or thousand Angstroms longitudinally and at least several hundred Angstroms laterally, especially for the samples imaged after the electrochemical cycling indicated.At the same time, the orientations of the row-like structures with respect to the X axis remained constant over distances of at least hundreds of Angstroms. Here, it is believed that the ordered row-like structures on the surfaces of the sample electrodes either before or after the electrochemical cycling in a sulphuric acid electrolyte may reflect, at least in part, the more ordered or crystalline regions of the polymer film^.'^,^^ Furthermore, the array of the branch-like fibrils illustrated in Fig.2(a) is attri- buted to an amorphous region typical of those which are generally assumed to surround the more crystallinedomain^.^^.^^ Assuming then that there are local crystalline regions on the surfaces of the polymer films, in principle it might be expected that individual pendent pyridine rings would be observed. In spite of much effort in the ordered areas of both the fibrillar and the granular rows, no topographical details as fine as a pyridine ring were resolved. This difficulty of imaging smaller scale molecular features is probably made more difficult by the inherent atacticity of the polyvinylpyri- dine matrix and the random loading of the osmium centres, but note that better ultimate resolution may be attainable in vacuum.On the other hand, in a study of the metallopolymer of Ru(bipy), bound to poly-4-~inylpyridine,~' a structural model was proposed in which a metallopolymer strand containing metal sites would exhibit a rod-like shape, especially at high metallation levels, because of the stereochemical and electro- static demands of the then present poly(pyridy1) ruthenium units. For the metallopolymer derived from Os(bipy),Cl, bound to poly-4-vinylpyridine, the diameter of the pendent osmium-containing unit is ca. 12-14 A. If the polymer back- bone is included, a total diameter is given in that direction of ca. 14-16 A.This would naturally be a minimum figure without consideration of the presence of chloride ions, hydrated or otherwise (diameter of the hydrated C1- ion is ca.7 A,26 that of free Cl- ion is 1.84 A). Replacement of chloride ions with sulphate ions during the electrochemical cycling in the electrolyte specified could also contribute to the effective diameter of the osmium complex polymer chains. Here, the spacing of rows of the straight fibrillar structure indicated in Fig. 2(a) and 2(b)is ca. 20 A,commensurate with the transverse dimension of the osmium-loaded polymer fea- tures outlined above. It is therefore tempting to reach the conclusion that the rows of straight fibrillar structure may represent the metallopolymer strands containing the active osmium sites.But considering the changes in the spacings of rows of granular structure or the separations of the wide fibrils after the electrochemical cycling used, it is also possible that the row-like structures are polymer backbones or a bundle of a few polymer chains, as suggested by the other authors.13-15,17,18 In regard to the terrace steps observed, the corresponding theoretical or experimental crystalline parameters available for comparison with the observed step heights shown in Fig. 2(c), 4(b) and 5(c) appear not to be known. However, if the osmium groups present are here assumed to somehow interlink with the immediate subsurface layer, the increase in the thickness as a result of the presence of the complexed osmium grouping would be ca.6-7 8,, the radius of the osmium pendent unit. This is very close to the measured step heights of 8-9 8, in Fig. 2(c). On this basis, the steps shown possibly represent extra polymer layers. On the other hand, the steps on the sample surfaces [see Fig. 4(b) and Fig. 5(c)], after electrochemical cycling, are found to have increased in step height and step edge-to-edge distance. However, there still remain certain regularities, for example, the step heights were usually multiples of ca. 20 or ca. 30 8,. The exact reason for such an increase is not clear at present. The concomitant exchange of chloride ions with sulphate ions may be signifi- cant. Moreover, the similar topographical features identified, especially the approximately parallel edges, of the steps seen are supportive of their common origin. Another significant change in the topography on the sample surfaces after electrochemical cycling used is the appearance of rows of nearly the same size as the observed granular structure, typically ca.40 8, x 40 8, or ca. 80 8, x 80 A, respect-ively [see Fig. 3(b) and Fig. 5(b)].Therefore, certain relation- ships appear to exist not only between the spacings of the row-like structures and the granular size but also between the granular size and the granular centre-to-centre distance. This is thought to be, in some way, related to the electrochemically induced process or processes which, in the presence of the sulphuric acid electrolyte, bring about the observed topo- graphical changes in the polymer matrix.Upon immersion of the polymer layer in sulphuric acid two processes are expected to occur. With the high acid concentrations, protonation of the free pyridine rings occurs immediately since the pK, of PVP is 3.5.25 Such a process is expected to alter the structure of the layer as the number of ionic sites will be greatly increased. A second significant change, as indicated above, will be the replacement of chloride ions by sulphate ions. These changes are expected to affect the layer structure significantly. It follows therefore that the layer granular structures observed might be caused by clus- tering of the osmium pendent units. It should be noted in this regard that XPS studies of the same materials, to be reported elsewhere,27 provide further evidence in support of these arguments. For example, the sulphate loading in the dried samples reflects the acid concentration used in the electrolyte, just as does the parallel development of protonated nitrogen centres.Furthermore, in the case of the ruthenium-containing metal- lopolymer [R~(bipy)~(PVP),(N0,)1 the+-coated ele~trode,’~ equilibrium spatial separation of the ruthenium centres is calculated to be 13.5 8, on the assumption of a rod-like structure. Also, in the study of a series of osmium and ruthenium-containing metallopolymers showing redox behav- iour, the suggestion is made that the average separation between the metal centres will be ca. 15 Accordingly, the granular structure observed here would not represent simply the redox or metal centres, disregarding the fact that the J.MATER. CHEM., 1991, VOL. 1 separation of the metal centres is connected with the nature of the polymer backbone and the metal-to-polymer ratio. In the images described, the granular sizes and the granular centre-to-centre distances were found to depend to a certain extent on the concentrations of the sulphuric acid electrolyte used. This behaviour is thought with some confidence to reflect the effect of the electrochemical cycling taking place in the osmium-containing metallopolymer.22 Now, there are some significant difficulties associated with deciding whether the granular structure shown in Fig. 3(b) and Fig. 5(b) rep-resents the redox clusters related to the osmium pendent units, or say the packing of the redox centres or something else.18 For example, there are few serious studies on the surface electronic properties and the crystalline structure of osmium-containing or analogous metallopolymers.Nevertheless, it is clear at this juncture that the granular structure observed arises from the effects of the electrochemical cycling in or on the surface of the osmium-containing metallopolymer-modi- fied polycrystalline electrodes concerned. 5. Conclusions Topographical images have been obtained from the osmium- containing metallopolymer-modified polycrystalline graphite electrodes by scanning tunnelling microscopy in air at room temperature. Before electrochemical cycling in a sulphuric acid electrolyte, rows of the fibrillar structure and significant steps of consistent size observed were found to be the main topographical features present on the sample surfaces.After electrochemical cycling, the topography of the sample surfaces was found to have changed in two ways. First, rows of distinct granules are observed, separately from an overlapping layer structure on the sample surface. Secondly, the steps observed are changed in both step height and step edge-to-edge distance. This change in topography is assumed to be associated with the attendant electrochemical cycling in the sulphuric acid electrolyte. 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