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Surface-Mediated Formation of Vanadium Hydrides R. Dus´,* E. Nowicka, and Z. Wolfram Institute of Physical Chemistry, Polish Academy of Sciences, ul.Kasprzaka 44/52, 01-224 Warszawa, Poland Received February 10, 1998. In Final Form: July 10, 1998 Surface phenomena which occur in the process of vanadium hydride VHy formation were studied through simultaneous measurements of surface potential and hydrogen pressure during H2 interaction “in situ” with thin vanadium films deposited under UHV conditions. The measurements were performed over large intervals of pressure and temperature from 10-10 to 10-2 Torr and 78-298 K, respectively. Two distinct forms of hydrogen adsorbate that occur on the surface in the course of the reaction were found, namely, (i) a negatively polarized β- form, which appears at the beginning of adsorption, and (ii) an induced, positively polarized β+ form, which is a precursor state for VHy formation. The kinetics of the β+ adspecies incorporation from the surface into the bulk was examined. Their equilibrium population on the surface with dependence on the hydrogen concentration in the bulk of VHy was determined.
Introduction Numerous studies have been performed to determine hydrogen adsorption states on surfaces of transition metals.1 Up to now, however, little is known in respect to surface phenomena occurring in the process of transition metal hydrides (MeHy ) formation, despite the increasing importance of these materials for hydrogen storage and catalysis.2 Transition metal hydrides are nonstoichiometric compounds with hydrogen distributed along interstitial sites of the metal,3,4 held in place by a combination of covalent and ionic bonding.5,6 This involves a net of positively charged hydrogen species. It is well-known that hydrogen adsorbs dissociatively on surfaces of transition metals. In some cases H adatoms do not incorporate from the surface into the bulk (e.g., in the case of adsorption on Pt9) or incorporate very slightly (e.g., in the case of adsorption on Fe10 or Co11), while in others the incorporation is very fast and efficient, leading to hydride formation (e.g., in the case of adsorption on Pd,7 Ti,8 Nb12). The question arises whether a distinct form of hydrogen adspecies is required for a fast incorporation into the bulk, or if an adsorbate of a similar character is created in all cases but the rate of its penetration below the surface depends on the properties of the metal. The existence of a specific form of hydrogen adsorbate, which is a precursor for palladium or titanium hydride formation has been observed;7,8 however, generally, the problem has not yet been solved. It is well established that some of transition metals change their original lattice structure with increasing (1) Christmann, K. Surf. Sci. Rep. 1988, 9, 1. (2) Metal-Hydrogen Systems, Fundamentals and Applications; Noreus, D., Rundquist, S., Wicke, E., Oldenburg, R., Eds.; Proceedings of the Third International Symposium, Uppsala, 1992. (3) Hydrogen in Metals; Alefeld, G., Volkl, J., Eds.; Topics in Applied Physics; Springer: Berlin, Heidelberg, New York, 1978; Vol. 29. (4) Fukai, Y. The Metal-Hydrogen System; Springer Series in Material Science; Springer: Berlin, 1993, Vol. 21. (5) Nagel, H.; Goretzki, H. J. Phys. Chem. Solids 1975, 36, 431. (6) Stalin˜ski, B.; Coogan, C. K.; Gutowsky, H. S. J. Chem. Phys. 1961, 34, 1191. (7) Dus´, R.; Nowicka, E.; Wolfram, Z. Surf. Sci. 1989, 216, 1. (8) Dus´, R.; Nowicka, E.; Wolfram, Z. Surf. Sci. 1992, 269/270, 545. (9) Nowicka, E.; Wolfram, Z.; Dus´, R. Acta Phys. Pol. 1992, A81, 117. (10) Nowicka, E.; Dus´, R. Surf. Sci. 1984, 144, 665. (11) Dus´, R.; Lisowski, W. Surf. Sci. 1976, 61, 635. (12) Dus´, R. Surf. Sci. 1975, 52, 440.
hydrogen concentration in the process of hydride formation.3,4 However, the influence of these phenomena on the behavior of hydrogen adsorbate is unknown. The aim of this work is to study what is the influence of the increasing hydrogen concentration in the bulk of metal in the process of a hydride formation on the form of the adsorbate and the kinetics of its incorporation below the surface. For the present study the vanadium-hydrogen system was chosen since vanadium hydride exhibits several distinct phases that depend on the hydrogen-vanadium atomic ratio (H/V).13-20 These phases differ with respect to hydrogen distribution over the tetrahedral and octahedral interstitial sites in the metal lattice and overall structure. At low hydrogen concentration (R phase) H atoms are randomly distributed over mainly tetrahedral sites in the bcc vanadium lattice.13,14,17 With increasing hydrogen content up to H/V ∼ 0.5 some precipitates of an ordered solution arise (low-temperature η phase,16 and hightemperature β phase14). In the η and β phases H atoms are placed mainly in the octahedral sites. They move into one type of the octahedral sites (Oz sites) and as a consequence the crystal structure becomes body-centered tetragonal (bct).4 The superstucture reflections of the δ phase (0.5 < H/V < 0.67) were observed by electron diffraction19 and detected by calorimetry.20 In this phase H atoms are placed preferentially over the octahedral sites, but some exchange with tetrahedral sites also occurs. Vanadium dihydride (VH2) (γ phase) formation is associated with lattice transformation from bct to fcc. VH2 precipitates appear in the η or δ matrix at an average concentration H/V > 0.68,14 or according to other assessors at H/V > 0.82.21 In the γ phase H atoms are placed in the tetrahedral gaps. It has been observed by means of proton (13) Mealend, A. J. J. Phys. Chem. 1964, 68, 2197. (14) Asano, H.; Abe, Y.; Hirobayashi, M. J. Phys. Soc. Jpn. 1976, 41, 974. (15) Fukai, Y.; Kazama, S. Scrip. Met. 1975, 9, 1073. (16) Schober, T.; Carl, A. Phys. Status Solidi A 1977, 43, 443. (17) Lasser, R.; Schober, T. J. Less-Common Metals 1987, 130, 453. (18) Fukai, Y.; Kazama, S. Acta Metall. 1977, 25, 59. (19) Wanagel, J.; Sass, S. L.; Batterman, B. W. Phys. Status Solidi A 1972, 11, 767. (20) Asano, H.; Hirabayashi, M. Phys. Status Solidi A 1973, 16, 69. (21) Cantrell, J.S.; Bowman, R. C., Jr.; Attalla, A.; Baker, R. W. Z. Phys. Chem. 1993, 181, 83.
S0743-7463(98)00159-0 CCC: $15.00 © 1998 American Chemical Society Published on Web 09/15/1998
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relaxation time measurements that at H/V ∼ 0.75 a maximum hydrogen mobility in the VHy lattice occurs. This was attributed to hydrogen occupancy on two distinct octahedral sites.21 However, the exchange of H atoms between the octahedral and the tetrahedral sites may also be of importance. Experimental Section We have found in the course of some preliminary studies22 that VH0.6-0.7 can be easily obtained during thin vanadium film interaction with H2 under a pressure on the order of 10-2 Torr. This allows use of a UHV apparatus to obtain a thin V film with a clean surface and transform this metal “in situ” into VHy in a controlled manner. The Pyrex glass UHV system is capable of reaching routinely 1 × 10-10 Torr during thin V film deposition and allows work at H2 pressures up to 10-1 Torr in the course of VHy formation was used. To study surface phenomena in the process of vanadium hydride formation an experimental method capable of distiguishing between different hydrogen adspecies over a wide pressure and temperature intervals is required. A measuring circuit with a short response time which allows observation of the surface process dynamics is useful. Previous experience has shown that these requirements are fulfilled while measuring simultaneously surface potential (SP) and H2 pressure P changes during hydrogen interaction with thin transition metal films.7-12 By definition SP ) -∆Φ, where ∆Φ is the change of work function caused by adsorption. Precise SP measurements allow determination of the electrostatic features of hydrogen adspecies and elucidation of the dynamics of elementary surface processes that occur during hydride formation. Simultaneous H2 pressure recording allows correlation of these features with the gas uptake determined volumetrically. H/V concentration can be also calculated since the mass of the deposited thin V film and the area of its surface are known. Precise SP measurements were carried out by means of the static capacitor method.23,24 The static capacitor was made of Pyrex glass and consisted of two coaxial cylindrical electrods. The inner one, which served as a reference electrode, was prepared by melting a conducting layer of (SnO + Sb2O5) into the glass. This electrode was completly inert against H2 adsorption under our experimental conditions, as tested by means of thermal desorption mass spectrometry (TDMS). The reference electrode was moveable and could be lifted up during thin metal film deposition on the inner wall of the outer cylinder, repositioned for SP measurements. A thin V film was the active electrode of the static capacitor. It was formed by a complete evaporation of a fine vanadium wire (Ventrin, Grade I) of known weight, wound around a tungsten heater. Thin V film was deposited while the static capacitor was immersed in a liquid nitrogen bath. Next the film was sintered at 320 K for 20 min under UHV conditions. The geometrical area of the film was 1.35 × 10-2 m2 and its average thickness 4 × 10-8 m. Every experiment was performed using a fresh thin V film. The surface potential was continuously monitored using a fully automatic, sensitive (1 mV), short response time (0.1 s), high stability (1 mV/h) electronic circuit. Thus the apparatus allowed observation of adsorption-desorption processes that occurred within a short time (e.g., 1 s), and also slower surface reactions. Spectroscopically pure hydrogen, purified additionally by diffusion through a palladium thimble was used for VHy formation. H2 was introduced in successive, calibrated doses from a container of known volume into the static capacitor maintained at a constant temperature and isolated from pumping by means of a system of greaseless Dekker valves. The capacity of every dose was determined by measuring the hydrogen pressure in the container by means of a precise McLeod manometer. The ionization gauge, which was used to monitor pressure during thin V film deposition, was switched off in the (22) Dus´, R.; Nowicka, E.; Wolfram, Z. J. Alloys Compounds 1977, 253/254, 496. (23) Delchar, T.; Eberhagen, A.; Tompkins, F. C. J. Sci. Instrum. 1963, 40, 105. (24) Nowicka, E.; Dus´, R. Surf. Sci. 1984, 144, 665.
Dus´ et al. course of VHy formation to avoid H2 dissociation on the hot filament as well as the gauge pumping effect. These two phenomena would introduce a significant error in calculations of hydrogen uptake. H2 pressure in the static capacitor was recorded by means of an ultrasensitive, short response time, Pirani type gauge immersed in a liquid nitrogen bath, reading from 10-6 to 10-1 Torr. A typical H2 dose when introduced into the static capacitor and not adsorbed would increase pressure up to 10-3 Torr. Thus knowing the volume of the static capacitor (750 cm3) the consumption of hydrogen following every succesive dose could be precisely determined. The examination of time dependent features of SPi(t) and Pi(t) changes associated with the ith introduced H2 dose allowed studies of the kinetics of surface processes at a determined H/V ratio. Equilibrium was achieved after the ith introduced dose when d(SP)i/dt ) 0 and dPi/dt ) 0. Having equilibrium surface potential SPi,eq and equilibrium H2 pressure Pi,eq for every hydrogen dose, two important relations can be determined: (1) the surface potential isotherm
SPeq )
∑SP
i,eq ) f1(H/V)tot
i
and (2) the thermodynamic isotherm
Peq )
∑P
i,eq
) f2(H/V)tot
i
Here (H/V)tot denotes the average, total, hydrogen concentration in VHy calculated as a ratio of the total uptake of hydrogen to the total amount of vanadium atoms on the surface and in the bulk. On the basis of these two functions distinct forms of hydrogen adsorbate can be differentiated. Thermal desorption mass spectrometry (TDMS) was applied to determine the purity of thin V films. A blind experiment was performed to examine any possible presence of impurities on freshly deposited and conventionally sintered thin V films. The results were compared with the TD spectrum obtained during decomposition of thin VH0.7 film. The H2 pressure as a function of thin film temperature P ) F1(T) and the temperature as a function of time T ) F2(t) were simultaneously recorded. Knowing the course of functions F1(T) and F2(t), it is possible to calculate the amount of hydrogen desorbed. The result of these calculations can be compared with the amount of hydrogen consumed in the process of VHy formation determined volumetrically, as described above. This is a useful way to verify TDMS data. The mass balance agreement between experimentally determined and calculated hydrogen uptake was ∼5%. On the basis of the volumetric measurements it could be expected that the TDMS signal would be very high. For that reason in the experiment with vanadium hydride decomposition the Faraday cup was installed in the mass spectrometer instead of an electron multiplier.
Experimental Results 1. Sample Characterization. AFM images of thin V film deposited on a flat glass support under the conditions described above and thin VHy (y ) 0.75) film obtained in UHV apparatus are shown in Figure 1. The images were obtained in air. A distinct difference in the nanostructure of the thin V and VHy films is clearly visible. It is known that vanadium hydride formation is associated with an increase of the lattice constant of the original metal.3,4 This leads to a strong surface reconstruction of the thin film, as can be seen in Figure 1. The roughness factors (defined as the ratio of the real surface area to the geometrical area) determined on the basis of the AFM images were 4 and 14 for thin V and VHy films, respectively. The roughness factor characteristic of VHy was used for further considerations. The question arises whether the exposition of thin V and VHy films to air required for the application of our atomic force microscope drastically changes the nanostructure of the samples. The high reactivity of vanadium with oxygen is well-known.
Surface-Mediated Formation of Vanadium Hydrides
Figure 1. AFM images of (i) a thin V film deposited under UHV conditions at 78 K and sintered at 320 K and (ii) a thin VHy film obtained “in situ” in the UHV apparatus during H2 interaction with thin V film at 298 K. The image was obtained in air by means of a commercial microscope (Topometrix, Discover).
Figure 2. TDMS spectrum for VH0.7 decomposition.
It is obvious that an oxide layer is formed immediately when thin V film is exposed to air. However, it is known that at room temperature the oxide is not thicker than 3-5 nm and forms a passive layer protecting the rest of the metal against oxidation. For this reason we assume that the nanostructure of the thin V film was not changed strongly as a result of exposition to air, although a rearrangement of vanadium atoms on the surface certainly occurred. A similar problem arises with the nanostructure of thin VHy film. The TDMS spectrum for thin VH0.7 film decomposition is shown in Figure 2. Vanadium hydride was obtained at 298 K, then cooled to 78 K under H2 pressure 2 × 10-2 Torr, and next evacuated. Further thermal desorption was carried out. The spectrum consists of two peaks. The
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big one corresponds to vanadium hydride decomposition within the whole bulk of the thin film. The small peak is associated with hydrogen desorption from the surface of the hydride. One can notice in Figure 2 that the decompostion of the thin VH0.7 film increased the H2 pressure up to 1.5 × 10-4 Torr, while hydrogen desorption from the hydride surface gave a TD signal on the level of 8 × 10-6 Torr. However, it should be emphasized (as will be shown further) that a part of the hydrogen deposit desorbs from the vanadium hydride surface during evacuation at 78 K. Thus the hydrogen concentration on the VHy surface under H2 pressure on the order of 10-310-2 Torr is certainly higher than that determined during the TD experiment. In the course of the blind TD experiment with fresh, sintered thin V film, the largest registered peak (m/e ) 28) was on the level of low 10-8 Torr. Thus the population of impurities on fresh, sintered thin V film is at least 3 orders of magnitude lower than the population of hydrogen on a thin VH0.7 surface. 2. Surface Potential Changes in the Process of Vanadium Hydride Formation. Surface potential changes caused by the successive H2 doses introduced into the static capacitor with thin V films at temperatures of 78, 195, and 298 K are shown in Figures 3, 4, and 6, respectively. Figures are divided into sections A, B, C, and D to better illuminate the successive steps observed in the process of VHy formation. Arrows indicate H2 introduction. The ratio (H/V)tot, is marked at every section. The differences in SP behavior at various temperatures are clearly seen. An excellent reproducibility of the experimental results (every experiment was repeated 2 or 3 times) should be emphasized. 2.1. Vanadium Hydride Formation at 78 K. At 78 K, at the beginning of adsorption, H2 doses caused a rapid decrease of SP followed by a constant surface potential value observed over an extended time. This is associated with a fast formation of negatively charged hydrogen adspecies, which are stable on the outer surface of the thin vanadium film. These adspecies are herein named β-. Many successive H2 doses resulted in an essentially different SP behavior. Positive surface potential transients were recorded (Figure 3, sections A, B, C).This phenomenon was caused by creation of the positively charged, atomic, hydrogen adspecies (named herein β+) on the outer surface of the film, followed by their incorporation into the bulk. A large amount of hydrogen was consumed during this step of VHy formation, up to (H/V)tot ∼ 0.2. Yet Peq remained 10-6 Torr. Creation of the positively charged, atomic, hydrogen adspecies on the surface of transition metals precovered by hydrogen deposit was predicted in Grimley’s model.25 According to this model the increase in the hydrogen adsorbate population is accompanied by an increase in the magnitude of the splitting between induced localized electron states. There is a critical population above which the lower state merges into the metal conduction band. Thereafter, the donation of electrons from hydrogen adatoms into unoccupied states in the conduction band occurs. This leads to the formation of the positively charged hydrogen adspecies. The constant SP value established after finishing every transient becomes increasingly more positive with successive H2 doses. This corresponds to the remaining β+ adspecies on the outer surface. The shape of the positive SP transients was not the same throughout the course of (25) Grimley, T. B., In Chemisorption; Garner, Ed.; Butterworth: London 1957.
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Figure 3. SP changes during vanadium hydride formation at 78 K. H2 was introduced into the static capacitor with the thin V film in successive doses (marked by arrows). The figure is divided into sections for better presentation of reaction steps. The average concentration of hydrogen (H/V)tot in VHy is marked throughout.
VHy formation, but rather it started to change when (H/ V)tot > 0.15. Above this hydrogen concentration a maximum incorporation rate was observed not at the beginning of the SP transient but after some time lag (Figure 3, section C). The time lag was longer with every successive H2 dose, and when (H/V)tot exceeded 0.2, further hydrogen doses resulted in a monotonic SP increase, corresponding to the increasing β+ population on the outer surface (Figure 3, section D). Every successive dose then caused an increase in the gas-phase pressure read by our Pirani gauge. However, the pressure was not constant as was SP but decreased, indicating some further consumption of hydrogen at a constant surface concentration of the β+ adspecies. This phenomenon was observed for a few doses only, and the consumption ceased. At an H2 pressure of 9 × 10-3 Torr, at 78 K the resultant maximal (H/V)tot value was ∼0.30. This value is much lower than that expected for vanadium dihydride. One can suppose that the inhibition of the β+ incorporation into the bulk of VHy was caused by thermally activated phase transitions within the surface-subsurface region. Rapid evacuation of the static capacitor down to 10-8 Torr at 78 K resulted in a decrease of SP by ∼150 mV and is presumably associated with desorption of positively charged hydrogen adsorbate. Clearly, a high population of the β+ adspecies on the outer surface of VHy at 78 K requires H2 pressures on the order of 10-3 Torr. This indicates a very weak adsorption (to be described in a separate paper). Similar features of SP(H/V)tot and P(H/ V)tot were also observed at 87 K (liquid argon bath). At this temperature the maximal value of (H/V)tot was higher than at 78 K, approaching 0.6. 2.2. Vanadium Hydride Formation at 195 K. At 195 K (Figure 4) the first hydrogen dose introduced into
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Figure 4. Course of SP changes during VHy formation at 195 K. H2 was introduced in successive doses (marked by arrows) into the static capacitor with the thin V film. The figure is divided into sections for better presentation of the reaction steps. The average concentration of hydrogen (H/V)tot is marked throughout.
the static capacitor with a thin V film caused an effect somewhat similar to that observed at 78 K. A rapid decrease of SP was observed followed by a constant value of surface potential, except a small, 2 mV increase recorded after 1 min. However, several successive H2 doses caused an entirely different SP behavior. Negative SP transients were registered (Figure 4, section A and B). The equilibrium surface potential recorded when the transients stabilized was increasingly more negative with every successive dose (Figure 4, section A). When (H/V)tot exceeded 0.059 hydrogen, dosing did not result in any measurable SP change (Figure 4, section B). However H2 pressure within every dose introduced into the static capacitor was decreasing (see Figure 5), indicating hydrogen consumption at a constant surface concentration. The equilibrium value of hydrogen pressure was below 10 -6 Torr despite a large uptake. Surprisingly, at (H/V) tot > 0.456 successive H2 doses caused a monotonic decrease of SP (Figure 3, section C). At this step of the reaction Peq within every dose was successively increasing. The average concentration (H/V)tot ) 0.92 was achieved under Peq ) 1.75 × 10-2 Torr. Evacuation increased the SP by 40 mV, almost up to the level registered at the plateau of surface potential (Figure 4, section C). 2.3. Vanadium Hydride Formation at 298 K. At 298 K (and at 273 K) successive H2 doses caused a monotonic decrease of SP at the beginning of adsorption (Figure 6, section A). When (H/V)tot exceeded 0.06, further H2 introduction resulted in an increase of hydrogen uptake, as determined on the basis of the pressure decrease within every dose (see Figure 5), while, the SP value was constant (Figure 6, section B). This SP plateau was recorded until (H/V)tot exceeded 0.60. Successive H2 doses resulted in a
Surface-Mediated Formation of Vanadium Hydrides
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Figure 5. Course of H2 pressure changes caused by the introduction of hydrogen doses into the static capacitor in the process of thin VH0.7 film formation within the SP plateau at 195 and 298 K. The values of (H/V)tot are marked at every dose. Figure 7. SP isotherms for VHy at 78 and 87 K. The equilibrium H2 pressure within the interval covered by an ultrasensitive Pirani gauge is also shown. In the frames are shown the intervals of concentration the β+ adspecies desorbing due to isothermal evacuation at 78 or 87 K.
Figure 6. Course of SP changes during VHy formation at 298 K. H2 was introduced into the static capacitor with the thin V film deposited under UHV conditions in successive doses as marked by arrows. The figure is divided into sections for better presentation of the distinguished steps of the reaction. The average hydrogen concentration in VHy is marked throughout
monotonic decrease of SP corresponding to a further uptake of the gas, while Peq was successively increasing (Figure 6, section C). An average concentration of hydrogen in vanadium hydride (H/V)tot ) 0.76 was achieved under H2 pressure 1 × 10-2 Torr. Evacuation caused a rather small increase of SP by only 8 mV. 3. Surface Potential Isotherms. Surface potential isotherms obtained at 78 and 87 K are shown in Figure 7, while those obtained at 195 and 298 K are presented in Figure 8. In these figures the dependences of Peq on (H/V)tot are also shown. In the frames are shown the intervals of concentration the β+ adspecies desorbing due to isothermal evacuation at 78 or 87 K. Discussion Figures 3, 4, and 6 show a drastic difference in the course of SP changes during VHy formation at low versus higher temperatures. This difference is also clearly seen in surface potential isotherms presented in Figures 7 and 8.
Figure 8. SP isotherms for VHy at 195 and 298 K. The equilibrium H2 pressure within the interval covered by an ultrasensitive Pirani gauge is also shown.
At 78 K a strong increase of SPeq by 1700 mV was recorded, remaining that known for alkali metals adsorption on transition metals,26,27 while at 195 K a significant decrease of SPeq by ∼400 mV was registered. There are two concepts possible for explanations of SP features in the process of VHy formation at low temperatures (78 or 87 K) and within (26) Gerlach, R. L.; Rhodin, T. N. Surf. Sci. 1970, 19, 403. (27) Fehrs, D. L.; Stickney, R. E. Surf. Sci. 1971, 24, 309.
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the temperature interval 195- 298 K. According to the first one, which is most likely, the β+ adspecies are the precursor state for vanadium hydride formation over the entire applied temperature range. They appear to be of a similar physical nature as hydrogen dissolved in the bulk of vanadium, in contrast to the β- adspecies. Thus the incorporation of β+ species from the surface into the bulk is caused simply by a concentration gradient, without the necessity of changing the electron distribution between the adsorbent and adsorbate. Two essentially different sites are available on the vanadium film surface for the β+ adspecies: above and below the surface image plane (SIP). The position below the SIP is energetically preferable. At low temperatures the adsorption sites above SIP are occupied, and this leads to an increase of the positive charge density on the outer surface and consequently to a pronounced SP increase. A small activation energy barrier hinders placement of the β+ adspecies below the SIP. Thus even a small increase in the adsorption temperature would result in the β+ adspecies distribution above and below the SIP. For this reason the maximal SPeq value in the SP isotherm obtained at 87 K is lower than that observed at 78 K (see Figure 7). At 195 K and above the β+, adspecies are placed mainly below the SIP, inducing a negative charge on the outer surface, according to Smoluchowski’s model.28 This results in the observed SP decrease. The value of the negative SP signal depends on the density of the β+ adspecies positioned below the SIP. At a similar H2 pressure, under equilibrium conditions, this density should increase with decreasing temperature since adsorption is an exothermic process. Thus at 195 K the SP should be more negative in the surface potential isotherm than at 298 K. This is clearly seen in Figure 8. The β+ adspecies placed below the SIP can diffuse further into the bulk of vanadium (or vanadium hydride when H/V < 2). This leads to a decrease of the adsorbate density and can be observed as negative SP transients (Figure 4, sections A and B). Hydrogen diffusion in metal bulk cannot be registered by means of SP measurements because of electron screening. As mentioned in the Introduction, it should be expected on the basis of vanadium-hydrogen phase diagram that around (H/V)tot ) 0.5 a new phase appears. This is associated with an increasing density of the β+ adspecies below the SIP, and a corresponding decrease of SP (see Figure 8). Hydrogen mobility in vanadium is very high.3 The activation energy for hydrogen diffusion Ediff is as low as 4.5 kJ/mol of H2.30,31 This is one of the lowest activation energies known for hydrogen diffusion in metals. Thus at 298 K the equilibrium between the adsorbate and the bulk, within a single H2 dose, is quickly achieved and the accumulation of hydrogen adspecies below the SIP is not percieved (Figure 6). However, at 195 K the elemetary successive steps of the process: (i) the adsorption of the β+ adspecies below the SIP, thereby decreasing the SP, and (ii) diffusion into the bulk, where lowering the adsorbate density and increasing SP may be experimentally differentiated. Similar phenomena were observed in the process of TiHy formation.31 The second concept for explanation of the SP features in the process of VHy formation assumes that over a temperature range of 195-298 K the negatively charged hydrogen adspecies arise on the outer surface of vanadium, and incorporate into the bulk. However, a shortcoming (28) Smoluchowski, R. Phys. Rev. 1941, 60, 661. (29) Cantelli, R.; Mazzolai, F. M.; Nuovo, M. J. Phys. Chem. Solids 1970, 31, 1811. (30) Heller, R.; Wipf, H. Phys. Status Solidi A 1976, 33, 525. (31) Nowicka, E. Vacuum 1996, 47, 193.
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Figure 9. SP dependence on the β+ adspecies concentration on the outer surface of a thin VHy film at 78 K. The relation between hydrogen concentration on the outer surface and in the bulk of the thin VHy film at 78 K is also shown. The deposit of the β+ adspecies desorbing during isothermal evacuation at 78 K is shown in the frame.
exists: it is difficult to explain the influence of temperature on the inversion of the polarity of hydrogen adatoms binding with vanadium, required to place the adspecies into the bulk of the metal. On the basis of SP and P features observed in the course of VHy formation at 78 K, it is possible to determine the distribution of hydrogen between the outer surface and the bulk of the thin V film. Figure 3 shows that hydrogen adsorption on the vanadium surface following hydrogen introduction is fast. The SP and P maxima are achieved within seconds. It can be realistically assumed that at this temperature the SP change from hydrogen uptake of a single, small, dose is proportional to the change in the adsorbate population on the outer surface. Hydrogen uptake obtained within 5-6 s from introduction of the ith dose can be volumetricaly determined, and correlated with the simultaneously registered SPi(t). It can be concluded on the basis of the course of SPi transients (see Figure 3) that within this short period of time a nearly complete hydrogen deposit remains on the outer surface, since SPi (t ) 6 s) is close to the maximal observed SPi. Thus the SP signal corresponding to the well-determined number of H atoms adsorbed in the β+ form on the outer surface of the thin vanadium film can be determined for every successive H2 dose, and consequently, for every stage of the reaction of VHy formation. On this basis, the equilibrium population of the β+ adspecies achieved within every dose after finishing of the SP transient can be calculated. Knowing the geometrical area of the thin V film, the rougness factor and the density of V atoms on the surface (NVS ) 1.47 × 1015 V atoms/cm2, taken as the average density for the planes 100, 110, and 21132) one can determine a function: SP ) f3(NHS/NVS), where NHS and NVS denote the population of the β+ adspecies and vanadium surface atoms, respectively. One can simultaneously obtain a relation between hydrogen concentration on the surface (NHS/NVS) and in the bulk (NHb/NVb) under the equilibrium conditions: (NHb/NVb) ) f4 (NHS/ NVS ). Here NHb and NVb denote uptake of hydrogen dissolved in the bulk of a thin V film and the amount of V atoms in the bulk of this film, respectively. Both of these values can be calculated by knowing the total uptake of hydrogen (H/V)tot, NHS, NVS, and the mass of the thin V film. The determined dependences are presented in Figure 9. (32) Brennan, D.; Hayward, D. O.; Trapnell, B. M. W. Proc. R. Soc. (London) A 1960, 256, 81.
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There are important features to note from the graphs in Figure 9: (i) As a result of H2 interaction with the thin V film at 78 K, the surface concentration of hydrogen (NHS/ NVS) approaches 2, the value expected for vanadium dihydride. (ii) The bulk concentration of hydrogen in the process of VHy formation increases quickly with increasing surface concentration until reaching a value of 0.15. Further, the β+ adspecies are placed mainly on the surface. (iii) Besides a small uptake of hydrogen adsorbed in the β- form at the beginning of reaction, the function f3 is linear up to the ratio (NHS/NVS) ∼ 1.90, that is up to receiving the reversibly adsorbed β+ adspecies onto the VHy surface. For that reason the Helmholtz equation can be applied to calculate the normal component of the dipole moment µ0 of the β+ adspecies present on the outer surface of the film within 0.05 < (NHS/NVS ) < 1.90. According to this equation:
SP ) 4πµ0(NHS/F)
(1)
where F is the real area of the film, calculated from the geometrical area and the roughness factor. The calculated value µ0 ) 0.13 D is very high, indicating an ionic component in the binding of the surface compounds. This corresponds to the strong SP increase observed in the process of VHy formation at 78 K. A strong surface potential increase has been also observed in the formation of other transition metal hydrides, e.g., PdH0.97 or TiH1.9.8 Analysis of positive SP transients allows investigation of the kinetics of β+ adspecies incorporation from the outer surface into the bulk of a thin V film. Indeed, the transients may be considered as a relaxation process. Introduction of a given H2 dose results in sudden increases in the β+ adspecies population on the surface (see Figure 3). In response to this perturbation, the adsorbate-adsorbent system readjusts to the new equilibrium on the surface and in the bulk. Since the rate of the β+ adspecies incorporation below the surface strongly predominates the rate of the reverse process, the positive SP transients can be described by a first-order kinetic equation:
-d[β+]i/dt ) k[β+]i
(2)
where [β+]i denotes the time dependent concentration of the β+ adspecies on the outer surface of the thin V film achieved as a result of the ith H2 dose introduction, and k is the rate constant. Assuming proportionality between [β+]i and SPi within the ith hydrogen dose, one can write
-d(SP)i/dt ) k(SP)i
(3)
Examination of the SP transients shows that eq 3 fits the experimental results obtained at 78 K when (H/V)tot 0.10 the maximal rate of SP transients occurs after a time lag. Simultaneously, the rate of P decrease indicating the rate of hydrogen consumption is smaller. The observed time lag in SP transients is a result of the superposition of at least two processes: (i) β+ adspecies incorporation below the surface and (ii) hydrogen adsorption on the released surface sites. When the rate of the first process decreases due to the lower mobility of hydrogen in VHy than in pure vanadium, the second process can predominate if the sticking probability is sufficiently high. Finally a monotonic increase of SP is observed (Figure 3, section D). The diffusion coefficient cannot be determined within (H/V) concentration corresponding to SP transients since the boundary conditions required for solution of Fick,s equation (constant surface concentration) are not fulfilled. As seen in Figure 3 the time of SP transients at temperatures as low as 78 K are short, or in other words, the rate of the β+ adspecies incorporation below the surface is very high. This rate was slightly higher at 87 K (see Figure 10). The activation energy for β+ adspecies incorporation Einc was estimated by assuming a classic, exponential dependence of the rate constant on temperature: Einc ) 2.5 kJ/mol of β+ (or 5.0 kJ/mol of H2), at (H/V)tot < 0.10. This value reflects the above mentioned activation energy for hydrogen diffusion into the bulk of vanadium. It is not certain, however, how far the classic description of the low-temperature rate constant for the β+ adspecies incorporation is valid, since the tunneling mechanism can play an important role in this process7,8 in addition to the thermally activated motion. Thus the estimated Einc corresponds only to the temperaturedependent component of the total activation energy.
5494 Langmuir, Vol. 14, No. 19, 1998
Dus´ et al.
The estimated activation energy for β+ adspecies incorporation below the surface Einc can be compared with Ediff calculated on the basis of observed H2 pressure decrease within a single dose at 273 or 298 K. Hydrogen diffusion into a thin film in the static capacitor can be well described by solution of Fick’s equation of sorption onto an infinite plate. Under boundary conditions, (i) constant concentration c0 of the diffusing medium in the bulk of the plate of thickness h at time t ) 0 and (ii) constant concentration c1 on the surface at t > 0, the amount of the medium Mt diffusing into the plate within a time t is:33
Mt/Minf ) 1 -
∑n {8/(2n + 1)2π2} exp[-D(2n + 1)2π2/4h2] (4)
where Minf corresponds to the amount sorbed within an infinite time and D is the diffusion coefficient. Replacing the series by the first term, and remembering that Mt can be volumetrically calculated, we have
[P(t) - Peq]/[P0 - Peq] ) (8/π2) exp[-bt]
(5)
where P0 is the maximal hydrogen pressure achieved as a result of the ith dose introduction, and b ) π2D/4h2. Simultaneous measurements of SP and P allow precise determination of the required boundary conditions as concerns a constant surface concentration of the diffusing medium during the whole process. These requirements are fulfilled within the SP plateau interval seen in the SP isotherms at 195 and 298 K (Figure 8). It was found that eq 5 sufficiently describes the time dependent H2 pressure decrease within the above-mentioned (H/V)tot regions, as shown in Figure 11. The insert shows a comparison between the calculated (eq 5) and experimentally determined H2 pressure decrease. The activation energy for hydrogen diffusion in VHy can be calculated on the basis of (b) factor dependence on temperature. When the thermally activated hydrogen motion predominates over tunneling, the diffusion coefficient can be described according to the classical relation:
D ) D0 exp(-Ediff/RT)
(6)
The parameters D0 and Ediff estimated within a narrow temperature interval 273-298 K are 1.45 × 10-12 cm2/s and 4.3 kJ/mol of H2, respectively. The actvation energy estimated for hydrogen diffusion in the thin VHy film with 0.1 < y < 0.45 does not really differ from that known for hydrogen diffusion in the bulk of vanadium. However, the preexponential factor for hydrogen diffusion in VHy is smaller by several orders of magnitude than the value D0 ) 3 × 10 -4 cm2/s determined for hydrogen diffusion in vanadium.29,30 It is worth noting that Ediff for hydrogen diffusion in VHy does not differ significantly from Einc (33) Crank, J. The Mathematics of Diffusion; Oxford University Press: London, 1956.
Figure 11. Examination of H2 pressure changes registered as a result of successive hydrogen dose introduction at 298 and 273 K into the static capacitor with the thin VHy film. The average concentration of hydrogen in vanadium is marked. In the inset a comparison of hydrogen pressure decrease measured experimentally during H2 interaction with VHy within a single dose at 273 K (continuous line) with calculated P(t) course (crosses) is shown.
estimated above for the β+ adspecies penetration below the surface. This supports the concept concerning the precursor role of these adspecies in the process of vanadium hydride formation within the temperature interval 78-298 K. Conclusions 1. Two distinct forms of hydrogen adspecies arise on thin V film surfaces during its interaction with H2, namely, (i) A negatively polarized, surface stable, β- form arising at the beginning of adsorption, and (ii) an induced, positively polarized β+, which is an apparent precursor for adsorbate incorporation below the surface, leading to vanadium hydride formation. 2. The position of the β+ adspecies on the VHy surface depends on temperature. At low temperature (78 K) they are placed above the surface image plane, thereby strongly increasing SP, while at higher temperature they occupy a position below the SIP, decreasing SP. 3. H2 interaction with thin V film at low temperature (78 K), under pressure on the order of 10-3 Torr, leads to VH1.95-2.0 formation on the surface, while the average hydrogen concentration in the bulk does not exceed H/V ) 0.2. 4. Examination of the positive SP transients arising at low temperatures within a single H2 dose allows determination of the activation energy for incorporation of the β+ adspecies below the surface. LA980159D
