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Atomic Force Microscopy Studies of Thin Pd Film Response to Palladium Hydride Formation and Its Reaction with Oxygen R. Nowakowski* and R. Dus´ Institute of Physical Chemistry of the Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warszawa, Poland Received November 12, 2002. In Final Form: April 21, 2003 Surface behavior of thin Pd film in the course of palladium hydride (PdHx) formation under H2 pressure of 101 kPa at 298 K and its decomposition in Ar or O2 atmospheres was studied “in situ” by combined methods of a video technique and atomic force microscopy. The main effort was concentrated on the determination of the role of local defects on the surface in the process of PdHx formation and its catalytic reaction with oxygen. The formation of PdHx within a thin Pd film causes drastic local changes of the surface corrugation as a result of stress creation and its relaxation within the film. Local surface changes around active sites and evaluation of these changes occurring during successive cycles of H2/O2 titration are distinguished and discussed. A comparison of the studied phenomena observed on two significantly different substrates, glass with random distribution of local defects and well-ordered mica with steps and terraces, exhibits differences resulting from the influence of the substrate on the generation of surface defects within the palladium film. Consequently the distribution of active sites in the reaction with hydrogen on thin Pd films reflects the distribution of local point defects on the substrate surface. The applied experimental procedure gives the possibility to distinguish between active sites in nanometric resolution and their evaluation in the course of the studied processes.
Introduction The influence of the density and distribution of active sites on catalyst surfaces on the selectivity and efficiency of heterogeneous reactions is commonly recognized. For this reason a great effort has been made to characterize the nature of these sites and the mechanism of their action. A great variety of experimental methods, described in several monographs,1,2 such as electron spectroscopes (Auger electron spectroscopy and electron spectroscopy for chemical analysis), electron diffraction (low-energy electron diffraction and reflection high-energy electron diffraction), infrared spectroscopy, electron microscopy, work function changes in the course of a catalytic reaction, accompanied by mass spectrometry methods (secondary ion mass spectrometry and thermal desorption mass spectrometry), etc., have been applied in these studies. Recently important results have been coming from the application of scanning microscopies (scanning tunneling microscopy and atomic force microscopy (AFM)).3-12 These methods, allowing the observation of local surface sites * Corresponding author. Telephone: +48 22 6323221. Fax: +48 22 6325276. E-mail:
[email protected]. (1) In Surface Science Techniques; Walls, J. M., Smith, R., Eds.; Pergamon: Oxford, 1994. (2) In Introduction to Surface Physical Chemistry; Christmann, K., Ed.; Steinkopff: Darmstadt (Springer: New York), 1991 (3) Erlandsson, R.; Eriksson, M.; Olsson, L.; Helmersson, U.; Lundstrom, I.; Petersson, L. G. J. Vac. Sci. Technol. 1991, B 9 (2), 825. (4) Yeung, K. L.; Lee, K. H.; Wolf, E. E. J. Catal. 1995, 156, 120. (5) Tanaka, H.; Yoshinobu, J.; Kawai, M. Surf. Sci. 1995, 327, L505. (6) Niehus, H.; Achete, C. Surf. Sci. 1996, 369, 9. (7) Somorjai, G. A. Appl. Surf. Sci. 1997, 121/122, 1. (8) Rosenhahn, A.; Schneider, J.; Kandler, J.; Becker, C.; Wandelt, K. Surf. Sci. 1999, 433, 705. (9) Schildenberger, M.; Prins, R.; Bonetti, Y. C. J. Phys. Chem. B 2000, 104, 3250. (10) Bird, D. P. C.; deCastilho, C. M. C.; Lambert, R. M. Surf. Sci. 2000, 449, L221. (11) Kobiela, T.; Dus, R. Vacuum 2001, 63/1-2, 267. (12) Mitsui, T.; Rose, M. K.; Fomin, E.; Ogletree, D. F.; Salmeron, M. J. Chem. Phys. 2002, 117 (12), 5855.
with an atomic resolution within a large interval of gas phase pressure from ultrahigh vacuum (UHV) up to 100 kPa, significantly reduce the well-known “pressure gap” problem. They also remove uncertainty in the description of phenomena occurring at the local sites of nanometric dimensions on the basis of averaged data recorded over much larger areas. In the presented work we applied the AFM method for studying the structure of a thin palladium film surface “in situ” in the course of palladium hydride formation under hydrogen pressure ∼101 kPa at 298 K, decomposition of this compound due to the change of gas-phase composition from H2 to Ar or O2, and further successive cycling of gas-phase composition changes. The main effort has been concentrated on determining the role of local defects on the surface in the process of palladium hydride formation and its reaction with oxygen. It should be expected that the structure of the substrate surface influences the density and distribution of defects within the thin Pd film. For this reason two kinds of substrate were applied: glass with a random distribution of defects after etching13 and well-ordered mica with steps and terraces formed as a result of cleavage. In these studies it was important to combine a short response time method allowing the observation of the surface process kinetics with one ensuring high-resolution recording of the surface nanostructure at chosen steps of the reaction. For this reason a combination of video techniques (a recording illustrating the surface response to the gas-phase composition changes) with AFM was applied. The preliminary results of these studies (thin Pd films on glass) were presented at ECOSS 20 and published in the proceedings of this conference.14 (13) Nowakowski, R.; Kobiela, T.; Wolfram, Z.; Dus, R. Appl. Surf. Sci. 1997, 115, 217. (14) Nowakowski, R.; Grzeszczak, P.; Dus, R. Surf. Sci. 2002, 507510, 813.
10.1021/la020908y CCC: $25.00 © 2003 American Chemical Society Published on Web 06/28/2003
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The equilibrium hydrogen pressure over PdHx (0.2 < x < 0.6) at 298K reaches ∼1 kPa and drops to 10-4 Pa at just 78 K. Moreover, water formed in the course of the catalytic process closely covers the palladium surface at low temperatures, making successive titration impossible. Thus these reactions, important in catalysis, cannot be investigated using a UHV system. Experimental Section Thin Pd wires (Johnson-Matthey grade I) of diameter 0.1 mm wound around a tungsten wire of diameter 0.3 mm were thermally deposited under UHV conditions (10-7 Pa) on microscopic plates made of glass or on mica, maintained at 78 K. The glass plates were etched in “aqua regia” and rinsed in doubly distilled water; the mica was freshly cleaved. After thin film deposition, the samples were sintered “in situ” at 370 K for 20 min, cooled to room temperature, taken out of the UHV apparatus, and placed into the AFM reactor. The film’s thickness could be determined by means of AFM after appropriate scratching. The AFM reactor connected with the gas dosing system enables “in situ” AFM investigation during controlled flow of neutral gas (argon) and two reagents (hydrogen and oxygen). AFM scanning was performed using a commercially available system (TopoMetrix, TMX 2000) and standard Si3N4 tips. Simultaneously, continuous observation of the sample was performed through a glass window using a video camera mounted above the reactor. A detailed description of the sample preparation and the investigation procedure was presented recently.14
Results and Discussion Characterization of Thin Pd Films Deposited on Glass during the Cycles of Hydrogen-Oxygen Titration. The frames extracted from the video recording of direct camera observation of the Pd film surface during one cycle of H2-O2 titration are presented in Figure 1. The images show the same surface area under successively introduced (frames a-d) hydrogen and (frames e-h) oxygen. The area is easily recognized by two perpendicularly oriented cracks visible on the images as thin black lines, which were made on the film surface with a needle before placing the sample in the AFM reactor. The image of the original thin Pd film exposed to air during transfer of the sample from the UHV apparatus to the AFM reactor is seen in frame a. The surface of the palladium is undoubtedly oxygen covered. The Pd film around the cracks is bright. This is a consequence of light reflecting from the smooth surface of the film, since light source is located on the opposite side of the video camera. Evident changes in the film brightness are observed when hydrogen is introduced over the surface (frames b-d). The “black” spots on the film surface arise within several seconds (the time period between frames a-d is 3-4 s). The density of the black spots increases successively with the time of exposure of palladium to H2. The black spots correspond to the transition of the thin Pd film surface from a smooth into a rough one. The only possible reason for this is the formation of a palladium hydride compound of a greater lattice constant than the original metal.15 This structure disintegrates when hydrogen flow through the AFM reactor is changed into oxygen flow (frames e-h). As a consequence, a similar image is observed by video camera before and after one cycle of H2-O2 titration (compare images a and h). It was noticed that the process of the black spots’ disintegration was slower than their formation (the time required to obtain images e-h is 7 s). The H2O2 titration can be repeated many times, leading to similar behavior. The only difference between the successive cycles (15) Lewis F. A. In The Palladium-Hydrogen System: Academic Press: London, 1967.
Figure 1. Video camera observation of palladium film deposited on glass during one cycle of hydrogen-oxygen titration under (a-d, time period 3-4 s) hydrogen and (e-h, time period 7 s) oxygen atmosphere. Pd film thickness was 60 nm.
Figure 2. Comparison of the video camera observation of palladium film, deposited on glass, in hydrogen atmosphere after (a) first, (b) sixth, (c) eighth, and (d) tenth cycles of hydrogen-oxygen titration. Pd film thickness was 60 nm.
is the increase of the black spots’ density with the number of cycles. This is demonstrated by comparison of the images of the same surface area in a hydrogen atmosphere after several cycles: (Figure 2a) first, (b) sixth, (c) eighth, and (d) tenth. Despite their distinct changes, the hydrogeninduced structures disintegrate in an oxygen atmosphere in the same manner as presented in Figure 1. The
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Figure 3. AFM imaging of surface corrugation changes of palladium film deposited on glass caused by a hydrogen atmosphere. The pictures were obtained for different film thickness: (a) 60 nm, (b) 40 nm, (c) 20 nm.
transition from a rough surface of the thin film into a smooth one must correspond to palladium hydride decomposition due to the catalytic reaction of hydrogen with oxygen. The water formed on the surface in the course of this reaction desorbs at 298 K and is removed with oxygen flowing through the reactor. The video camera observations indicate that the black spot structures are formed randomly on the surface. It seems that the macroscopic defect induced on the film surfacesthe cracks made with a needlesdoes not play an important role. This suggests that the black spot structures are formed around defects of another kind (size) originally randomly distributed over the surface and those additionally produced due to the reaction. As a consequence, randomly distributed black spots arise on the surface in the hydrogen atmosphere and expand during further cycles of titration. We have observed a strong dependence of the size and form of the surface structures arising under the H2 atmosphere on the Pd film thickness. These structures are more pronounced when thicker films are used. In the case of very thin (lower than 20 nm) but continuous Pd films, the structures were not observed in our experiments. AFM studies were applied to determine the fine features of the black spot structures. With the video camera image of the surface in the hydrogen atmosphere, the AFM tip was located over the area covered by black spots and scanning was performed. The results obtained for three Pd films of different thickness are compared in Figure 3. In all cases the structures stick out from the surface. The differences in the structure size and form depending on the Pd film thickness are evident. In comparison with the original Pd film, stronger changes arise for thicker films (Figure 3a, film thickness >50 nm). The surface is much rougher and is characterized by the presence of a huge change of the film corrugation. The height of the hill-like protrusions is around 1-2 µm, which is 20-40 times higher than the thickness of the Pd film. The structures cover the whole area, giving the surface a “totally wrinkled” appearance. Despite these huge changes made by hydrogen within the thin Pd film, the wrinkled structures disappear when the atmosphere is changed to O2 and also to Ar. This means that thin palladium hydride film decomposition occurs easily not only during the catalytic reaction but also when hydrogen pressure in the gas phase drops below the equilibrium pressure. The changes of thin Pd film under the H2 atmosphere are more discreet for
thinner films. In this case not the whole surface is changed. The wrinkled structures form a mutually linked network sticking out above the unchanged area. This is observed for Pd film thickness between 20 and 50 nm (Figure 3b). For thinner films of thickness around 20 nm even single linear structures are detected (Figure 3c). The height of the structures in this case is hundreds of nanometers, while the width corresponds to a single micrometer. The change of the H2 atmosphere to Ar or O2 caused the disappearance of the described structures. The phenomena reported above were observed with excellent reproducibility. It was interesting to study the features of the single linear structures more precisely. The AFM images of the formation and disintegration of the same structure during one cycle of H2-O2 titration are presented in Figure 4. The images correspond to: (a) the structure in the hydrogen atmosphere, (b) disintegration of the structure shortly after changing the atmosphere to oxygen, (c) a trace of the disintegrated structure observed in the oxygen atmosphere, (d) formation of the structure shortly after hydrogen reintroduction, and finally (e) a stable structure rebuilt in the hydrogen atmosphere after the presented cycle of H2-O2 titration. In the course of our experiments the H2 pressure (102 kPa) strongly exceeds the equilibrium pressure (∼1 kPa) required for palladium hydride formation at 298 K.15 The creation of PdHx occurs within the Pd film, since the oxygen adlayer reacts quickly with hydrogen from the gas phase. Water, the product of the reaction, desorbs from the palladium at 298 K leaving a clean surface of thin Pd film active for reaction with hydrogen in the process of palladium hydride formation. The lattice constant of PdHx exceeds that of the original metal by ∼3.5%. The formation of PdHx in the H2 atmosphere under our experimental conditions has been proved by means of measurements of the resistance of thin Pd film changes. The change of thin Pd film resistance in the process of PdHx formation is well described in the literature.15 On the basis of Figure 4 it is evident that the structure arising on the thin Pd film surface under the H2 atmosphere consists of many mutually linked point spots. This is visible in the image Figure 4a as white stars. We shall call this structure a star structure. This observation indicates that the surface structure changes arising due to the interaction of the Pd film with hydrogen are initiated at the point defects. Palladium hydride is formed around these point
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Figure 4. AFM images of the same surface structure on palladium film (20 nm of thickness) deposited on glass observed during one cycle of hydrogen-oxygen titration: (a) the structure in hydrogen atmosphere; (b) disintegration of the structure after changing the hydrogen to oxygen; (c) a trace of the disintegrated structure obtained in oxygen; (d) formation of the structure after hydrogen reintroduction; (e) stable structure rebuilt in hydrogen after the presented hydrogen-oxygen titration cycle.
defects. This process leads to the formation of strain or stress fields in the film, which locally produce starlike protrusions. The distance between the observed active points reaches hundreds of nanometers to several micrometers. The stress is relaxed at a greater distance from the point source, leading to the formation of linear protrusions linking the active points. Consequently the large linear structure visible in the images is formed. As mentioned above, the structure decomposes when the atmosphere is changed to O2 or Ar (Figure 4b). It is interesting to note that decomposition is not complete. A weak trace of the structure after its partial decomposition in the oxygen atmosphere remains on the film surface (Figure 4c). The shape of this trace exactly reproduces the star structure recorded in the H2 atmosphere. The height of this protrusion is around 20 nm, which is 20-30 times smaller than the height of the structure in the hydrogen atmosphere. The question remains whether this locally observed film reorganization is a result of palladium hydride formation and decomposition or a consequence of the thermal effect associated with the formation of water on the film surface. It is well-known that water formation is a strongly exothermic process. Successive introduction of hydrogen again leads to the formation of the star structure (Figure 4d). Comparison of the star structures created in this titration cycle (Figure 4e) with those arising during the previous cycles (Figure 4a) shows the progress of the star structure formation in successive cycles of H2-O2 titration. The influence of water molecules on AFM imaging observed on Pt film during titration was discussed previously.14 Even more information concerning our system can be obtained when an additional kind of measurementsforce modulation techniquesis applied. This is the subject of additional studies and will be published separately. Characterization of Thin Pd Films Deposited on Mica during the Cycles of Hydrogen-Oxygen Titration. The observed star structure on thin Pd film in a hydrogen atmosphere is not a direct effect of PdHx formation. In the authors’ opinion it is a result of relaxation of the stress involved in the film. Two factors could therefore be expected to influence this process. The first one is the direct interaction of Pd with hydrogen leading to PdHx formation. As a consequence strain or stress fields are generated in the film. The second one is the film’s interaction with the substrate. This could influence the relaxation of stress within the film. Note that the phenomena described above were observed within a Pd film deposited on a glass plate. The surface of glass etched
in “aqua regia” is amorphous with a high density of defects.13 A strong stochastic factor should be taken into account in the relaxation process. In the authors’ opinion a comparison of the reported results with those expected for a more defined system with an atomically ordered substrate is justified for better understanding of the discussed phenomena. Thus, corresponding experiments were performed using Pd films deposited on mica. The mica substrate was chosen because it has a well-defined ordered surface and, like glass, is an insulating material. Figure 5 shows the video camera images of the same area of Pd film surface deposited on mica registered in (frame a) air and under successively introduced (frames b, d, and f) hydrogen and (frames c, e, and g) the neutral gas argon. The Pd film thickness was around 50 nm, which enables observed discontinuities of the film caused by the split of steps of the mica atomic layers (marked by black and white arrows in Figure 5a). The discontinuities of the film in these places are confirmed by resistivity higher than 108 Ω measured between opposite sides of the step. On the other hand, large areas from tens to hundreds of square micrometers without these defects exhibit a continuous layer of thin Pd film characterized by resistivity of around 40 Ω. The first change on the surface observed as a result of hydrogen introduction is the formation of a dark network on the bright light-reflecting surface (Figure 5b). Surprisingly, this structure in the main part stays on the surface regardless of successive changes of the gas atmosphere from H2 to Ar or O2 (Figure 5c,e,g). The reintroduction of hydrogen causes changes observed as dark spots within the network (Figure 5d,f). Contrary to the stable network, these changes are reversible and the dark spots disappear when the H2 atmosphere is changed to Ar or O2. The density of the dark spots successively increases with the number of H2-Ar or H2-O2 cycles. Finally, after several cycles of titration, the main part of the surface becomes dark in the hydrogen atmosphere, but always a change from H2 to Ar or O2 restores the stable dark network on the surface. These observations indicate that we are dealing with two different responses of thin Pd film deposited on mica for interaction with hydrogen: the formation of the stable network and the reversible structures within the network. The phenomenon presented in Figure 5 was observed simultaneously using the AFM method. Figure 6 shows AFM images of the same area of Pd film under successively introduced argon (frames a, c, e, and g) and hydrogen (frames b, d, and f). This area is recognizable by a step boundary located in the left part of the images (frame a). The introduction of hydrogen leads to the formation of
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Figure 5. Video camera observation of the same area of palladium film deposited on mica in (a) air and during successively introduced (b, d, f) hydrogen and (c, e, g) argon (black and white arrows correspond to the mica atomic layer steps).
mutually linked linear protrusions observed in the video camera as a black network (frame b). The height of the
structures is between 1 and 1.5 µm, while the width corresponds to 2-3 µm. It is interesting to analyze the
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Figure 7. Continuation of the AFM observation presented in Figure 6 obtained under successively introduced: (a) oxygen, (b) hydrogen, (c) argon, (d) hydrogen, and (e) oxygen atmosphere.
Figure 6. AFM images of the same surface area of palladium film deposited on mica obtained during hydrogen-argon cycling: under successively introduced (a, c, e, g) argon and (b, d, f) hydrogen atmosphere (white arrow corresponds to the mica atomic layer steps).
progress of the structures located near the step boundary. The step boundary is characterized in hydrogen by the highest protrusion marked on the image with a white arrow. Evidently the structures close to the boundary are oriented perpendicularly to the step, the distance between them is around 25-45 µm. The network is formed at some distance from the boundary (30-50 µm). The observed structures in the main part stay on the surface when the H2 atmosphere is changed to Ar or O2 (Figure 6c). The reintroduction of hydrogen leads to the evolution of star structures within the network (Figure 6d,f), a process similar to that observed on Pd deposited on glass. Contrary to the stable network, these changes are reversible and disappear in the argon (or oxygen) atmosphere (Figure 6e,g). Let us turn our attention to the network. Although the network structure is stable regardless of the gas atmosphere, some differences between hydrogen and argon (oxygen) are observed. The first difference is the height which is much greater in hydrogen. A comparison of the same place indicates that the height changes from 1.6 to 0.9 µm. The second variation is the shape of the protrusions. As opposed to the straight-line shape of the network
structure observed in the argon or oxygen atmosphere, the network structure in hydrogen additionally shows internal oscillations (period equal to 5 µm). In the authors’ opinion this observation is important since it confirms the stress generated within this structure during PdHx formation. Although the main feature of thin Pd film response to changes from H2 to both Ar and O2 is the same, whether following PdHx decomposition due to the decrease of H2 pressure below the equilibrium pressure or a surface reaction resulting in water formation, careful inspection of the images indicates some differences. This can be seen comparing Figures 6 and 7. Figure 7 presents the successive steps of the experiment, but instead of Ar, in this case O2 was introduced (Figure 7a). The introduction of oxygen after argon did not change the topography of the surface (compare Figure 6g and Figure 7a). However differences in the film response were observed when H2 was introduced after oxygen (Figure 7b) in comparison to the hydrogen flow performed after argon (Figure 6f, Figure 7d). In the first case the film changes are weaker and mainly concern the growth and extension of the network. No characteristic star structure is formed in this case. This observation indicates that adsorbed oxygen stabilizes the surface of thin Pd film making it less reactive for PdHx formation than in the case when Ar, which does not adsorb on palladium, is present in the gas phase. Interaction of hydrogen with palladium covered by oxygen consists of two processes: water formation on the surface followed by slow desorption of the product at 298 K, and the
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Figure 8. The network structure within palladium film deposited on mica observed in air after palladium hydride formation by optical microscope in transmission mode (black arrow corresponds to the mica atomic layer steps).
subsequent reaction with palladium leading to PdHx formation. This second reaction is obviously restricted only to the part of the surface which is not covered by water at the time of image formation. The time of data aquisition was the same for the Ar and O2 atmosphere. The described formation of the network structure was observed on a large area. Figure 8 shows the images of large area obtained by means of an optical microscope in transmission mode. The dark network is distinguishable on the surface, confirming that the network structure is stable in air. The dark line in the upper part of the image corresponds to the split of several steps of the mica atomic terraces. It is evident that the arms of the network around this discontinuity are oriented perpendicularly to the steps (marked with a black arrow on the zoom image in Figure 8). This observation opens the question of the mechanism of the network structure formation. Figure 9 shows the set of frames collected every successive 0.6 s using a video camera during “in situ” observation of thin Pd film reacting with hydrogen. The network structure arises around active points of the film located on the steps of the substrate (see two different places marked in Figure 9a,b with black arrows). The networks born in these areas mutually join (frame c) and extend over the surface (frames d-f). It is worth noting that the extension of the network structure runs symmetrically on both sides of the step (although a step is not symmetrical, having a lower and upper side). Consequently the described phenomena lead to the interesting process of linking of two network structures generated from two different steps. However this phenomenon is not presented in the figures. It proceeds in the same way, but on a much larger area, like the process shown here around the single step. A comparison of the images in frames b and c of Figure 9, where the linking of two network structures generated in two different areas around the step, marked with black arrows, illustrates the features of this process.
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Figure 9. Video camera observation collected during every successive 0.6 s, showing formation of the network structure within palladium film in hydrogen atmosphere (black arrows correspond to two different areas around the step where the network formation is simultaneously initiated).
Figure 10. AFM images of the same area of Pd film deposited on mica collected during hydrogen-argon cycling in four successive steps of hydrogen introduction (successively a, b, c, d) (white rings in (a) correspond to the area where the star structures were formed (b); white arrow in (b) marks the star structure from which the network structure is built (c), white arrows in (c) marks the newly induced active sites which react with reintroduced hydrogen causing evolution of the network structure (d)).
More detailed observation of the mechanism of the network structure formation is undertaken by means of AFM. Figure 10 shows a set of four images of the same area of thin Pd film collected during H2-Ar cycling in four successive steps of hydrogen introduction. In the course of the first two cycles, PdHx generation occurs around
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separated sites, leading to the formation of star structures in active places (see places marked by white rings (frame a) and the corresponding star structures arising in the next cycle (frame b)). Contrary to the active sites on the flat part of the film, which are separated by rather long distances (up to tens of micrometers), the active points located on the step have a tendency to form chains consisting of several sites in the near neighborhood (shown in the bottom part of image b). We can conclude that the local concentration of active sites is higher around the step than over the rest of the film. During successive H2-Ar cycles two different processes occurred on the Pd film in a hydrogen atmosphere. The first is the linking of neighboring active points by linear protrusions. As a result, a network structure is formed (see the star structure marked with a white arrow in image b and the corresponding network structure generated from this point marked in image c with a gray arrow). In the authors’ opinion this process is a consequence of relaxation of the stress field generated in the active points due to the PdHx formation. The presented mechanism is also compatible with the observation that the network around the step is oriented perpendicularly since the formation of linear protrusions occurs between the nearest located active points. The second process is observed simultaneouslys the star structure formation around newly induced active sites (see the star structures marked with white arrows in image c). These newly induced active sites react during the successive introduction of hydrogen causing evolution of the network structure (d). This confirms that PdHx formation is initiated at the point defects of the Pd film. Such defects tend to be located near the steps of the mica substrate producing discontinuities within the thin Pd film. During successive H2-Ar(O2) cycles, the tendency of the film toward relaxation of the stress field leads to the formation of a stable, irreversible network structure and induces generation of new active sites (point defects) around which PdHx formation occurs. As a consequence, the evolution of the stable network structure over the whole area of the PdHx film can be distinguished, in the first step, and the generation of reversible star structures within the network can be distinguished in the second step. Comparison of the Response of Pd Film Deposited on Glass or Mica Caused by Film Interaction with Hydrogen. The results discussed above enable us to summarize the influence of the substrate on thin Pd film behavior during interaction with hydrogen. Some similarities and differences can be distinguished. The first important observation which is common for films deposited on nonordered (glass) and ordered (mica) substrates is the fact that the first changes of film topography in a hydrogen atmosphere occur at point defects. In both cases local protrusions, starlike structures, are formed (see Figures 3c and 10b,c). The results indicate a similar mechanism of evolution of these structures during the next cycles of titration, leading to the formation of a structure of mutually linked protrusions; however the scale of this process is significantly different. Evidently, this difference results from the much higher density of active sites (observed in hydrogen as star structures) on the film evaporated on glass. This is confirmed by a comparison of films of similar thickness on glass (Figure 3b) and mica (Figure 10). The stress generated at all the point defects due to PdHx formation is relaxed between the active points in the form of linear protrusions. Consequently, the resulting structures of mutually linked protrusions differ significantly in size. Since the films were prepared in the same manner, the question arises about the reason for
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the difference in the density of active sites. The most likely reason is the influence of the substrate. Films deposited on ordered, highly homogeneous substrates (mica) are characterized by a lower number of defects. Only a step boundary, which is a linear defect, produces a greater number of active sites from which the formation of the network begins (these are chains of point defects, observed in hydrogen as chains of star structures). On the other hand a rougher substrate (glass) generates a higher concentration of defects on the thin film surface. The local density of active sites is higher for thinner film and decreases with the increase of film thickness (the distance between the star structures around the linear protrusion, observed for thinner film (Figure 3c), is smaller than that measured for thicker film, where the structure of mutually linked protrusions is formed (Figure 3b)). This observation seems to confirm the role of the substrate in generating defects on the thin film surface, which decreases with the increase of film thickness. The second difference concerns the behavior of the star structures and network structures in argon (also in oxygen). Contrary to the star structure generated on the film deposited on glass, the network structure generated on mica is not reversible. Two reasons for this behavior could be taken into account. The first is a consequence of the fact that the network structure is a result of stress relaxation within the film. Since the mesh of the network generated on mica is significantly larger than the dimension of the protrusion structure on glass (compare Figures 3b and 6c), higher stress is expected within the film as a result of it cumulating from a larger area. The higher stress could exceed the level of elastic deformation, leading to plastic, irreversible changes of the surface. The second reason results from the influence of the substrate itself on the process of PdHx formation and the shape and size of the generated network. It is known that cleavage of mica in air is accompanied by electrical charge.16 This strong electrostatic interaction influences the cleaved surface’s topography.17 In our opinion the generation of stress within mica can also be expected. Consequently, the strong diffusional relaxation of the stress present in the film as a result of PdHx formation in a strain gradient generated within the substrate can be taken into account (Gorsky effect18-20). According to this mechanism, the irreversible network observed here results from a different mechanism (focused stress relaxation in a gradient of strain) and visualizes the spectrum of existing stress distribution in the film/ substrate system. It is interesting to note that the network structure generated on the Pd film deposited on mica often consists of a hexagonal mesh whose shape correlates with the shape of a unit cell of mica (observed by AFM in atomic resolution). Summary The combined techniques of AFM and video have been applied to “in situ” observation of thin Pd films during reactions with gas phases: hydrogen and oxygen. Hydrogen interaction with Pd films causes drastic changes of the surface corrugation as a result of the creation of stresses within the film during PdHx formation. The observation of this phenomenon can be summarized as follows: (16) Poppa, H.; Elliot, A. G. Surf. Sci. 1971, 24, 149. (17) Vancea, J.; Reiss, G.; Schneider, F.; Bauer, K.; Hoffmann, H. Surf. Sci. 1989, 218, 108. (18) Peisl, H. In Hydrogen in Metals I; Alefeld, G., Voelkl, J., Eds.; Springer-Verlag: Berlin, 1978; p 53 (19) Gorsky, W. S. Z. Phys. SU 1935, 8, 457. (20) Schaumann, G.; Voelkl, J.; Alefeld, G. Phys. Rev. Lett. 1968, 21, 891.
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Langmuir, Vol. 19, No. 17, 2003
(1) The role of point defects in the distribution of local sites active in the process of PdHx formation within thin Pd film deposited on glass or mica is evidently observed. The kind (size) of defects is important in this process (macroscopic defects induced on the film as cracks play a weaker role in comparison to microscopic defects). (2) The influence of the substrate surface on the amount and distribution of point defects of thin Pd film is shown. Local sites active in PdHx formation are randomly distributed along the glass surface. This is a consequence of the random distribution of defects on the substrate surface. A much smaller number of active sites is observed on thin film deposited on mica. They are concentrated mainly along the step boundary of the substrate surface. (3) The influence of the substrate on the number and distribution of point defects on the film surface, and consequently active areas in reaction with hydrogen, decreases with the increase of film thickness. (4) The surface changes generated around active sites due to PdHx formation (star structures) disintegrate in the neutral Ar atmosphere (due to the decrease of hydrogen pressure below the equilibrium level) or in oxygen (catalytic reaction of hydrogen and oxygen). However, careful inspection indicates that the structure disintegration in O2 atmosphere is not total and a weak trace of the
Nowakowski and Dus´
structure remains. This observation suggests irreversible film reorganization during H2/O2 titration. (5) Evolution of the surface changes from active places occurred during further cycles of H2/O2 titration, leading to the formation of structures mutually linking the star structures. This process proceeds differently on glass and on mica as a consequence of different density of active sites: a structure of mutually linked protrusions and a network structure are formed, respectively. (6) Contrary to the structure of mutually linked protrusions (also the star structure) observed on the glass substrate, the network structure generated on mica is not reversible. This observation confirms the differences in the mechanism of structure formation. The discussed investigation is an indirect visualization of the mechanism of palladium hydride formation. Because the used procedure allows distinguishing active sites and their evolution on a relatively rough surface (thin films), the results presented here could be interesting with respect to the optimization of metal surfaces during hydride formation. This is an important point since, as described above, only specific defects create active sites in the reaction with hydrogen. LA020908Y
