Surface-Mediated Thin Terbium Hydride Film Formation - Langmuir

Jan 15, 2010 - TbHx (x ≈ 3) formation resulted in ΔΦ = ∼200 mV and an increase in the thin film area by a factor of ∼3. These phenomena were ...
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Surface-Mediated Thin Terbium Hydride Film Formation Marek Knor, Robert Nowakowski,* Ewa Nowicka, and Ryszard Dus Institute of Physical Chemistry of the Polish Academy of Sciences, ul. Kasprzaka 44, 01-224 Warszawa, Poland Received August 17, 2009. Revised Manuscript Received December 23, 2009 This work was performed to study the correlation between the surface and bulk phenomena that occur during H2 interaction with terbium, leading to three-hydride TbHx (x ≈ 3) formation. This reaction is accompanied by the transition of the original metal into a semiconductor. It was found that thin films are particularly useful for such studies. Measurements of work function changes ΔΦ(H/Tb) were chosen to illustrate the surface phenomena, and the relative electrical resistance R(H/Tb)/R0 and light transparency T(H/Tb)/T0 correspond to the bulk properties. Additionally, BET experiments were performed to determine the influence of three-hydride formation on the area of a thin Tb film. It was observed at 298 K that a precursor state of the adsorbate arose at the beginning of the reaction, when (H/Tb < 0.1), decreasing the work function by ΔΦ = 12 mV. A higher uptake of hydrogen caused an increased work function, followed by ΔΦ transients. This has been interpreted as local hydride formation on the surface and its expansion into the bulk, until a concentration of H/Tb ≈ 3 was reached. TbHx (x ≈ 3) formation resulted in ΔΦ = ∼200 mV and an increase in the thin film area by a factor of ∼3. These phenomena were accompanied by characteristic changes in the bulk properties. The light-reflecting thin Tb film was transformed into a transparent hydride, with an∼ 23-fold increase in R/R0. At 78 K, only a small amount of hydrogen (H/Tb = 0.13) was consumed, leading to ΔΦ = -23 mV. This uptake is stable up to 100 K. Increasing the temperature above this value resulted in the additional large absorption of hydrogen. This could suggest the formation of a low-temperature surface phase of the hydride.

Introduction It has been well established that H2 interaction with hexagonal rare earth metals Me leads to the exothermic formation of hydrides MeHx (x ≈ 3) if the required thermodynamic conditions are reached.1-3 The hydrides are nonstoichiometric compounds with hydrogen located along the interstitial sites of the original metal. In the process of MeHx formation, the crystallographic structure of the original metal changes depending on the hydrogen concentration. In the case of TbHx (0 < x < 3), three phases (R, β, and γ) have been distinguished so far and a fourth one, the low-temperature R* phase, could be expected:3-5 (i) The R phase (0 < x < 1) is a solution of hydrogen in terbium. It exhibits metallic character. Hydrogen atoms are located in some of the tetrahedral intersites of the hexagonal host metal lattice. (ii) The β phase (1 < x < 2) is also metallic, but its crystallographic structure is changed to fcc. H atoms occupy two available tetrahedral sites in the lattice unit cell. (iii) The γ phase (2 < x < 3) is a semiconductor with hcp structure but with lattice constants different from those of the original metal. Hydrogen atoms are placed in one octahedral and two tetrahedral intersites. The metal-semiconductor transition that occurs in the process of trihydride formation is accompanied by drastic changes in the material’s physical properties. The binding energy of hydrogen in the γ phase is much lower than in the β phase. Hence reducing or increasing the H2 pressure in the reactor results in the decomposition *Corresponding author. E-mail: [email protected]. (1) Arons, R. R. In Rare Earth Hydrides; Hellwege, K. H., Ed.; Landolt-Bornstein New Series; Springer-Verlag: Berlin, 1982; Vol. 12c, Chapter 63. (2) Arons, R. R. In Rare Earth Hydrides; Wijn, H. P. J., Ed.; Landolt-Bornstein New Series; Springer-Verlag: Berlin, 1991; Vol. 19d1, Chapter 23. (3) Vajda, P. Hydrogen in Rare Earth Hydrides. In Handbook on Physics and Chemistry of Rare Earth Metals; Gschneidner, K. A., Jr., Eyring, L., Eds.; Elsevier Science B. V.; Amsterdam, 1995; Vol. 20, Chapter 137. (4) Blaschko, O.; Krexner, G.; Daon, J. N.; Vajda, P. Phys. Rev. Lett. 1985, 55, 2876. (5) Mc Kregow, M. W.; Ross, D. K.; Bonnet, J. E.; Anderson, J. S.; Schaerpf, O. J. J. Phys. C: Solid State Phys. 1987, 20, 1909.

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or formation of trihydride. This offers the possibility to work with materials of very different physical properties. For example, metallic light-reflecting thin TbHx (x < 2) film can be transformed into a transparent semiconductor of composition TbHx (2 < x < 3). Metal f semiconductor (or insulator) transitions of rare earth metals, often referred to as switchable mirror formation,6,7 are still intensively studied because of their possible application in electro-optical devices. The majority of investigations have been concerned with bulk properties,1-3 but little is known about surface phenomena. The aim of this work was to find a correlation between the elementary steps of surface processes occurring in the course of TbHx (0 < x < 3) formation and bulk property changes registered during this reaction. It has been found that the application of thin terbium films is convenient for this purpose. Work function changes registered depending on hydrogen atomic concentration in metal ΔΦ(H/Tb) were chosen to illustrate surface phenomena, whereas changes in the relative electrical resistance R(H/Tb)/R0 and light transparency T(H/Tb)/T0 characterized the bulk. Terbium is a highly reactive metal, thus UHV conditions on the order of 10-8 Pa are required to prepare a thin Tb film with a clean surface. It is well known3 that trihydride formation induces stress that disintegrates the bulk sample into powder. This can be avoided, however, when thin films are applied.6 Nevertheless, even for thin films one should expect protrusions induced by stress to arise within the MeHx (x > 2) films. These protrusions can increase the area of the films and influence the work function.8 The Modification of grain boundaries can also occur, changing the resistance and transmittance.5 BET experiments were performed to estimate the enlargement of the area of thin Tb films. While investigating diffuse neutron scattering, it was observed that at low temperatures (6) Huiberts, J. N.; Griessen, R.; Rector, J. H.; Wijngaarden, R. J.; Dekker, J. P.; de Grott, D. G.; Koeman, N. J. Nature 1996, 380, 231. (7) Dus, R.; Nowicka, E. Surf. Sci. 2002, 819, 507–510. (8) Dus, R.; Nowicka, E.; Nowakowski, R. J. Alloys Compd. 2005, 404, 284.

Published on Web 01/15/2010

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(∼78 K) in several MeHx systems (Me = Sc, Y, Ho, Er, and Lu) hydrogen in the R phase exhibits a tendency to order in the subsurface region.4,5 This ordered phase, stable at low temperature and referred to as R*, strongly inhibits the penetration of H atoms into the bulk. Thus, hydrogen consumption is very small. It has been reported that small hydrogen uptake that could be reached at 78 K on yttrium does not result in a measurable change in the work function,9 as suggested, because of the presence of the H-Y-H adspecies with a very small dipole moment on the yttrium surface. However, H2 interaction with a thin Gd film at 78 K resulted in a small but measurable ΔΦ signal.10 To our knowledge, there is a lack of data concerning the existence and properties of the R* phase in terbium. This is why surface phenomena occurring at low temperature during hydrogen interaction with the thin Tb film were studied as well as those produced at 298 K.

Experimental Section UHV System. The experiments were performed using a UHV glass system capable of routinely reaching the basis pressure of (1-2)  10-8 Pa during thin Tb film preparation. At the same time, the apparatus offered the possibility of studying H2 interaction with terbium within the pressure interval of 10-7-100 Pa during TbHx (0 < x < 3) formation. Remember that in glass UHV systems evacuated by mercury diffusion pumps nitrogen is the main component of the residual gases, which is much larger than H2. This allows us to minimize any noncontrolled hydrogen adsorption on the sample surface, an important requirement when a small uptake of H2 is being determined. Two types of glass reactors were applied: (i) a spherical one for measurements of the thin film resistance, transparency, and BET experiments11 and (ii) a cylindrical static capacitor cell for determining work function changes.12,13 Tb Film Deposition, Electrical Resistance, and Optical Transparency Measurements. A thin film was obtained by the complete evaporation of strips of Tb foil (Alfa Aesar, Johnson Matthey GmbH, 99.9%) of known weight (8.27 mg), spot welding to a tungsten wire (heater), and deposition on the wall of the reactor, maintained at a constant temperature. Spectroscopically pure hydrogen (Edelgase, Leipzig, Betrieb Berlin, Wasserstoff reinst) was used and was additionally purified in the course of the experiment by diffusion through a palladium thimble. H2 was introduced into the reactor, disconnected from pumps by means of a system of greaseless Dekker valves in successive calibrated doses, with pressure measured by means of a precise McLeod gauge. Because the volume of the reactor and the tubing was carefully determined, the uptake of hydrogen and the atomic H/Tb ratio resulting from every dose could be calculated. Hence, all studied properties of the sample on the surface or in the bulk can be expressed as functions of the average H/Tb atomic ratio. The spherical reactor was equipped with connections for measuring thin film resistance prepared by semimelting onto the inner wall of two strips made of platinum foil, and spot welding with a platinum wire, which was joined to the tungsten feedthroughs. The resistance measured continuously during H2 interaction with the sample could be determined within the interval of 10-3-1010 Ω. Original Tb films with a resistance of 70-90 Ω were applied. This corresponds to an estimated average thickness of around 30 nm. Transmittance, which is inversely proportional to the beam of light attenuated by the thin film, was determined by means of a photodiode. BET Measurements. The adsorption of Xe at 78 K on thin Tb and TbHx (x ≈ 3) films was investigated in the course of BET (9) Dus, R.; Nowicka, E. Langmuir 2000, 16, 10258. (10) Nowicka, E.; Nowakowski, R.; Dus, R. Appl. Surf. Sci. 2008, 254, 4146. (11) Dus, R.; Nowicka, E.; Nowakowski, R. Langmuir 2004, 20, 9138. (12) Delchar, T.; Eberhagen, A.; Tompkins, F. C. J. Sci. Instrum. 1963, 40, 105. (13) Nowicka, E.; Dus, R. Surf. Sci. 1984, 144, 665.

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experiments to determine the surface area of the films. The equilibrium pressure of liquid xenon at 78 K reaches P0 = 0.226 Pa, and the area occupied by one Xe atom on the surface was taken to be AXe = 1.7  10-15 cm2.14 To examine how far our BET data are realistic, a blind experiment was performed to determine the area of the spherical reactor in the absence of a thin metal film (205 cm2) and to compare the result with the geometrical area calculated on the basis of the known relation between the volume and the surface area of the sphere and cylindrical tubing, measuring the weight of the water that filled the reactor. The agreement was within 2%. The commonly known BET equation can be written in the convenient form P 1 ðC - 1ÞðP=P0 Þ ¼ þ N Xe ðP0 - PÞ CN m CN m

ð1Þ

where P is the xenon pressure in equilibrium with the adsorbate containing NXe xenon atoms, Nm is the number of Xe atoms in the monolayer, and C is a constant. The area of the adsorbent S can be calculated according to the obvious relation: S = AXeNm. Work Function Changes. Surface phenomena in the process of rare earth metal hydride formation require an experimental method capable of distinguishing between various hydrogen adspecies over a wide pressure and temperature interval, around 10-7-100 Pa and 78-300 K, respectively. Precise ΔΦ measurements allow the electrostatic features of the adsorbate to be determined and the dynamics of elementary surface processes occurring during hydride formation to be elucidated. Simultaneous pressure measurements allow the correlation of these features with the uptake of hydrogen determined volumetrically. The use of an adsorbent with a large surface area and a cell with a small volume is suggested, as well as the application of an electronic circuit of high sensitivity and short response time. Among a few experimental techniques meeting these requirements, we chose the static capacitor method.12,13,15 The construction of the cell and electronic circuit and the calculation method have been described in detail elsewhere.13,15 An electronic circuit with a sensitivity of 1 mV and a response time of 1 ms allowed the continuous measurement of work function changes in situ in the course of TbHx formation.15 H2 was introduced in successive calibrated doses into the capacitor disconnected from pumps by means of a system of greaseless valves and maintained at a constant temperature, with pressure measured following each dose introduction when d(ΔΦ)/dt ≈ 0. Hence, by knowing the effective volume of the static capacitor, the uptake of hydrogen could be calculated on the basis of volumetric data. The relation ΔΦ = f (H/Tb)T was also determined and combined with features concerning the bulk sample.

Experimental Results and Discussion In the first step in our studies, we have to prove that under our experimental conditions a metallic thin Tb film can be transformed into TbHx (2 < x < 3). This can be done by comparing the bulk features of our sample influenced by hydrogen with those described in the literature for terbium hydride. Volumetric measurements clearly indicate that TbHx (2 < x < 3) can be easily obtained at room temperature under H2 pressure on the order of 10 Pa. Figure 1 shows photographs of the reactor used for resistance measurements and for the determination of the thin films’ optical properties. Figure 1a corresponds to the “empty” (without thin Tb film) glass reactor made of Pyrex glass, Figure 1b presents the reactor with a metallic, light-reflecting thin Tb film, and in Figure 1c the reactor contains a thin TbH2.85 hydride kept at 298 K under H2 pressure of 30 Pa. One can easily notice that this thin film is transparent. The described optical properties agree with those known for the β f γ transition in hydrides MeHx (x > 2) of the rare earth hexagonal metals.6,7,10 We found that the (14) Roberts, R. H.; Pritchard, J. Surf. Sci. 1976, 54, 687. (15) Bachtin, A. Vacuum 1985, 12, 519.

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Figure 2. Course of time-dependent changes in the resistance and relative optical transparency of the thin Tb film caused by successive hydrogen doses introducted into the reactor maintained at 298 K. Division into sections a-c better illuminates the progress of the reaction. The interval of the H/Tb ratio corresponding to every section is marked.

Figure 1. Change in the optical transparency of the thin Tb film during TbHx (0 < x < 3) formation: (a) an empty spherical reactor (without a thin Tb film), (b) a reactor with a metallic, light-reflecting thin Tb film, and (c) a reactor with a transparent thin TbH2.85 film. electrical resistance of the original thin Tb film decreases with decreasing temperature, as should be expected for metallic terbium. However, reducing the temperature of the thin TbH2.85 film from 298 to 78 K resulted in a 3-fold increase in resistance. The negative temperature coefficient of electrical resistance indicates that thin TbH2.85 film behaves like a semiconductor, as should be expected. The kinetics of the resistance and relative transparency changes in the thin Tb film caused by successive hydrogen doses in the process of terbium hydride formation at 298 K is shown in Figure 2. 3304 DOI: 10.1021/la904205p

The Figure is divided into three sections (a-c) to visualize the progress of the reaction better. The interval of the H/Tb atomic ratio corresponding to every section is marked. One can notice in Figure 2 that the first few doses of H2 resulted in a small, almost stable increase in resistance, but the transparency is not changed. This is ascribed to a hydrogen-induced increase, on the surface, of the population of scattering centers for conduction electrons. The number of hydrogen atoms responsible for this effect approaches (3-5)  1017, while the total number of terbium atoms in the film reaches 3.5  1019. An increase in resistance due to the hydrogeninduced enlargement of the population of the scattering centers for electrons on the surface and in the subsurface region has been theoretically considered and experimentally observed for several metals.16-18 A number of successive H2 doses, however, caused resistance transients: an increase in R followed by a decrease. We suggest that this corresponds to the penetration of the adsorbate from the surface into the bulk. In this way, accessible hydrogen adsorption surface sites are liberated for reaction with H2 from the successive doses. The increase in R in transients caused by successive H2 doses is, however, progressively smaller and finally starts not to be measurable. Simultaneously, the thin Tb film exhibits transparency that progressively increases with every successive H2 dose. It should be expected that the nature and the population of the scattering centers on the surface depend on the hydrogen concentration in the subsurface region. It is not well understood why hydrogen present in the bulk thin Tb film within the atomic (16) Wissman, P. Springer Tracts Mod. Phys. 1975, 77, 1. (17) Wedler, G. In Chemisorption: An Experimental Approach; Butterworths, London, 1976. (18) Dus, R.; Nowicka, E. Prog. Surf. Sci. 2003, 74, 39.

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Figure 4. Examination of the BET relation according to eq 1 for the original thin Tb film (curve a) and for the thin TbH2.85 film (curve b).

Figure 3. Dependence of the relative resistance ΔR/R0 and the relative optical transparency ΔT/T0 on the average H/Tb atomic ratio. Every point in the graph corresponds to the equilibrium reached after successive H2 dose introduction. ratio interval of 0.15 < H/Tb < 1.7 decreases the resistance. A combination of several effects could play a role: (i) a modification of the contact between thin film grains caused by reaction-induced stress, (ii) a change in the population of the scattering centers for conduction electrons in the bulk, resulting from the increase in hydrogen concentration, (iii) a change in the carrier concentration due to the donation of electrons from hydrogen atoms into the conduction band of the metal. It has been well established that hydrogen in the bulk of several metals, such as Pd, Ni, and V, exhibits protonic character19 whereas hydrogen in yttrium threehydride behaves like an anion.20 The problem concerning the change in the nature of hydrogen in the bulk of rare earth metals with increasing H/Tb concentration is not trivial and needs more experimental and theoretical work. For TbHx (x > 1.8), every successive H2 dose resulted in a strong increase in resistance. Under a hydrogen pressure of 30-50 Pa, terbium three-hydride TbHx (x ≈ 3) can be routinely obtained. The good reproducibility of the described results is worth noting. A course of R(H/Me) similar to that described here has also been observed for yttrium and gadolinium.9,10 Figure 3 presents the course of the equilibrium resistance and transparency as a function of the atomic H/Tb ratio. The transparency of the thin Tb films changed only slightly with the increase in hydrogen concentration up to the H/Tb ratio of around 0.7. This suggests that within the aforementioned hydrogen uptake, modification of the grain size, which could change the contact between the grains and thus influence the light transparency and also the resistance of the thin film deposited on glass, is not very significant. At higher H/Tb ratios, the light transparency strongly increases with every successive H2 dose up to 1.7. One can notice in Figure 3 that within a narrow hydrogen concentration interval in TbHx (1.7 < x < 2) some decrease in thin film transparency is observed. This is probably a result of terbium hydride grain reorientation induced by stress within the thin film. Later, within TbHx (2 < x < 3), the transparency gradually increases, accompanied by an increase in resistance, until a limit associated with three-hydride formation is reached. BET experiments clearly indicated an increase in the thin Tb film area induced by stress in the course of three-hydride (19) Brouwer, R. C.; Griessen, R. Phys. Rev. Lett. 1989, 62, 1760. (20) den Broeder, F. J. A.; van der Molen, S. J.; Kremers, M.; Huiberts, J. N.; Nagengast, D. G.; van Gogh, A. T. M.; Huisman, W. H.; Koeman, N. J.; Koeman, N. I.; Dam, B.; Rector, J. H.; Plota, S.; Haaksma, M.; Hanzen, R. M. N.; Jungblut, R. M.; Duine, P. A.; Griessen, R. Nature 1998, 394, 656.

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formation. Figure 4 shows the results of the experiments presented according to eq 1. The thin TbHx film has an area that is around 3 times larger than that of the original thin Tb film: 660 and 207 cm2. We assume that the reported increase in film area in the process of hydride formation in the thin Tb film deposited on glass is caused by stress generation, its relaxation, and the resulting formation of protrusions. Such a change in thin film topography was clearly determined on the basis of AFM data for hydride formation within thin palladium film.8 Direct proof of the same phenomenon for terbium requires the application of the appropriate AFM system, which is not available in our laboratory. H2 interaction with the thin Tb film at 78 K did not result in a measurable increase in resistance, whereas some small consumption of hydrogen was clearly detected. This uptake could correspond to the aforementioned low-temperature, ordered R* surface phase. Having the area of the thin Tb film and the uptake of hydrogen, we can estimate the thickness of the R* phase. For this purpose, the average concentration of Tb atoms on 1 cm2 of the thin film surface can be roughly estimated as the average density of terbium atoms on planes (111), (100), and (211)21 reaching 9.5  1014 cm-2. It is reasonable to expect that this hydrogen deposit consists of H-Tb-H species, as in the case of the ordered low-temperature phase in yttrium. Calculations produce ∼4 H-Tb-H layers under equilibrium H2 pressure of around 1 Pa. If the R* phase really exists at the surface, then it should decompose at a defined temperature, making it possible for hydrogen to penetrate into the bulk. Moreover, the H-Tb-H species should exhibit a very small (or nonmeasurable) dipole moment, thus leading to a very small change in the work function. The first condition can be examined by measuring the resistance of the thin Tb film maintained at 78 K in a reactor filled with hydrogen under a pressure of around 10 Pa, disconnected from pumps and with gradually increasing temperature. Figure 5 shows the result of such an experiment. The presented behavior of resistance (graph a) is corrected by subtraction of the dependence of R on temperature for the thin Tb film in vacuum and thus concerns the net effect of H2 interaction with the thin terbium film. The stability of the R* phase up to 100 K is clearly visible. At higher temperatures, the resistance changes are due to the decomposition of the R* phase and the incorporation of hydrogen into the bulk. The complex course of the ΔR(T) function is caused by a variation of the hydrogen penetration rate below the surface, caused by a decrease in H2 pressure in the closed reactor during the described process and ΔR approaching equilibrium in the bulk after the consumption of all of the gas in the reactor. The total H2 content in the closed reactor during the course of the experiment described above corresponded to the ratio H/Tb =1. (21) Brennan, D.; Hayward, D. O.; Trapnell, B. M. W. Proc. R. Soc. London, Ser. A 1960, 58, 81.

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Figure 5. Influence of temperature on the resistance of the thin Tb film interacting with H2. Curve a was obtained by subtracting curve b from curve c, hence corresponding to net ΔR(T) changes caused by H2 interaction with the thin Tb film. Curve b shows the temperature dependence R(T) of the clean thin Tb film under UHV conditions. Curve c represents the resistance changes ΔR(T) of the thin Tb film (maintained primarily in vacuum at 78 K in the spherical reactor) resulting from the interaction with a large dose of hydrogen during the course of the temperature increase.

Figure 7. Dependence of work function changes of the thin TbHx film on the H/Tb average atomic ratio at 298 K. Courses of timedependent ΔΦ(t) values caused by the introduction of single successive H2 doses (marked by arrows) are shown in the inset.

the well-known Helmholtz equation ΔΦ ¼ 4πμ0 ns Θ

Figure 6. Dependence of work function changes of the thin Tb film on hydrogen uptake at 78 K. The course of time-dependent ΔΦ caused by successive H2 dose introduction is shown in the inset. Work function changes caused by the interaction of successive H2 doses with the thin Tb film at 78 K as a function of hydrogen uptake ΔΦ(ΝH2) are shown in Figure 6. The inset presents the course of ΔΦ(t) registered during this process. A small, monotonic decrease in the work function is observed as the result of the introduction of several successive doses, leading to a total ΔΦ decrease of 23 mV under H2 pressure of 3 Pa. Further increases in hydrogen pressure resulted in small additional uptake determined volumetrically, without any ΔΦ signal. It should be expected that because of electron screening in the bulk of the metal, the adsorption-induced work function changes are effected by the variation of charge distribution caused by adspecies present on the surface within one to three layers of the adsorbent atoms. The adsorbate-located dipper in the subsurface disappears from ΔΦ detection. Isothermal evacuation did not alter the work function, indicating a stable hydrogen deposit. It can be estimated that the R* phase in the described experiment extends over 10 layers of Tb atoms. Deviation from the result described above ΔR(T) is not very large. A linear dependence of ΔΦ on hydrogen uptake allows the calculation of the normal component of the dipole moment of hydrogen adspecies μ0 on the surface of the R* phase according to 3306 DOI: 10.1021/la904205p

ð2Þ

where ns is the density of surface sites available for adsorption and Θ is the coverage. Knowing the population of the adsorbate Nads based on volumetric measurements and the adsorbent area S determined by BET experiment, while remembering that Θ = Nads/Sns, we can write 4πμ0 N ads ð3Þ S It is reasonable to assume that the adsorbent consists of H-TbH species on the surface, a situation similar to that suggested previously for hydrogen adsorption on yttrium and gadolinium.9,10 Calculated from eq 3, the normal component of the dipole moment of hydrogen adspiecies on the R* phase surface reaches μ0 = 6.0  10-3 D. The ΔΦ(H/Tb) function obtained during the course of the experiment carried out at 298 K differs greatly. The work function dependence on the H/Tb atomic ratio is shown in Figure 7a. Parts b and c present the features of ΔΦ(t) courses characteristic of the selected intervals of hydrogen concentration. At the very beginning of the process, a few H2 doses decrease the work function, as observed at 78 K. This uptake is higher than that corresponding to a monolayer and approaches H-Tb-H formation within around 10 terbium layers. At a slightly higher hydrogen deposit, every successive dose resulted in positive ΔΦ transients. (A decrease in the work function is followed by its increase.) This corresponds to the ΔR transients mentioned above. After that, many successive hydrogen doses caused reverse ΔΦ transients. (An increase in the work function is followed by a decrease.) A large amount of hydrogen ΔΦ ¼

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Knor et al. up to H/Tb ≈ 3 is consumed in this way. The transients are well described by the first-order kinetic equations. We suggest that the phenomenon observed at a high H/Tb ratio results from a convolution of several factors: (i) adsorption with the creation of negatively charged adspecies on the surface; (ii) their slow incorporation into the bulk, accompanied by TbHx formation; and (iii) the change in thin film topography induced by stress. By studying work function changes during palladium hydride formation, we noticed that the stress induced by this reaction, arising in the thin film deposited on glass, leads to the creation of protrusions.8 This strongly influences work function changes. BET results clearly showed an increase in the terbium hydride area in comparison with the original thin Tb film. Hence, the presence of stress should certainly be expected. The total change in the work function being a superposition of the above-mentioned effects is not very high. At H/ Tb ≈ 3, under H2 pressure of 30 Pa, the work function increases by ∼200 mV. Isothermal evacuation resulted in a decrease in the work function corresponding to the desorption of negatively charged adspecies from the surface and some decomposition of terbium hydride at a high H/Tb ratio. We suggest that the observed course of ΔΦ(H/Tb) corresponds to the change in charge-transfer direction during terbium hydride formation. This is associated with the change in the hydrogen adsorbate nature. At H/Tb < 0.5, positively polarized adspecies are formed with a decrease in the work function. At 298 K, this deposit is quickly incorporated into the bulk of the thin Tb film, but below 100 K, it is stable within the subsurface region, forming the R* phase. At 298 K, increases in hydrogen concentration in the subsurface region and in the bulk changes also conditions for adsorption on the surface. Negatively polarized hydrogen adspecies are now created increasing the work function. Their slow incorporation into the bulk creates the hydride. This is accompanied by the stress that influences ΔΦ.

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Conclusions (1)

(2)

(3)

(4)

(5)

(6)

TbHx (0 < x 1.7. TbH2.85 exhibits a negative temperature coefficient of electrical resistance characteristic of semiconductors. Changes in bulk properties determined for the thin Tb film in this process are correlated with surface phenomena. At low coverage, hydrogen deposition decreases the work function of thin Tb films at both temperatures (78 and 298 K). Higher uptake, however, increases the work function. This observation corresponds to the change in charge-transfer direction in the adsorbate-adsorbent system. The presence of a low-temperature R* phase within the subsurface of the thin Tb film is suggested on the basis of the experimental data. This phase is stable below 100 K.

Acknowledgment. This work was supported by the Polish Ministry of Education and Science through grant no. NN204 241634.

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