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Surface Science 600 (2006) 4113–4118

The initial stages of the hydrogen-induced reconstructionof Pd(110) studied with STM

Marko Kralj *, Conrad Becker, Klaus Wandelt

Institut fur Physikalische und Theoretische Chemie, Universitat Bonn, Wegelerstr. 12, D-53115 Bonn, Germany

Available online 6 May 2006

Abstract

The hydrogen-induced reconstruction of the Pd(110) surface was investigated in situ with scanning tunneling microscopy (STM).Focusing on the initial stages of the restructuring, which ultimately leads to a stable (1 · 2) reconstructed surface, we find an exponentialincrease of the reconstructed surface area with hydrogen exposure, up to 8 Langmuir, which can be explained by an autocatalytic behav-ior. Moreover, the steps, especially those running along the [001] direction, play a distinctive role in the buildup of the (1 · 2)reconstruction.� 2006 Elsevier B.V. All rights reserved.

Keywords: Scanning tunneling microscopy; Surface relaxation and reconstruction; Palladium; Hydrogen

1. Introduction

The Pd(1 10) surface has received considerable interestdue to the broad use of palladium as a heterogeneous cat-alyst and its potential use in hydrogen storage devices. Inparticular, the adsorption and interaction of oxygen andhydrogen with Pd(110) have been subject of numerousinvestigations. Early low energy electron diffraction(LEED) studies by Conrad et al. revealed that the exposureof the Pd(110) surface to hydrogen leads to a (1 · 2) recon-struction [1]. Following this early report hydrogen adsorp-tion on Pd(1 10) has been the subject of quite a fewexperimental and theoretical studies. The theoretical calcu-lations show that there is only a slight surface energy differ-ence between the unreconstructed and the reconstructed(110) surface, which suggests that both structures may bepresent on the clean surface [2,3]. In spite of that, a recon-structed clean Pd(110) has only been reported once [4].Later it was shown with scanning tunneling microscopy

0039-6028/$ - see front matter � 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.susc.2006.01.130

* Corresponding author. Permanent address: Institute of Physics, P.O.Box 304, Zagreb HR-10000, Croatia. Tel.: +49 228 732537; fax: +49 228732915.

E-mail addresses: mkralj@uni-bonn.de, mkralj@ifs.hr (M. Kralj).

(STM) [5] that already small amounts of hydrogen leadto a reconstruction of the Pd(110) surface. Furthermore,the Pd(110) surface is one of the examples where a richset of stable and metastable hydrogen adsorption configu-rations can be related to a number of experimentallyobserved reconstruction phases [5–7]. Theoretical calcula-tions have shown that the most stable hydrogen adsorptionconfiguration leads to a (1 · 2) missing-row reconstructedsurface. It was demonstrated that the gain in hydrogenadsorption energy suffices to stabilize the reconstructedsubstrate. The (1 · 2) missing-row reconstruction is accom-panied by the creation of (111) micro facets, which providethe most stable adsorption sites for hydrogen atoms [3].

In this report we revisit the problem of the room temper-ature hydrogen induced missing-row reconstruction of thepalladium surface focusing on the very initial stages. Theinteraction of hydrogen with Pd(110) is studied in situfor various exposures and dosing conditions in UHV byuse of STM. In order to accomplish this a very low residualhydrogen pressure in the low 10�9 Pa region was crucial.The STM enabled us to study the very initial stages ofthe local palladium reconstruction in great detail, whichare not accessible to averaging surface structure sensitivetechniques, e.g. low energy electron diffraction (LEED)

(b)

[001]

[110]

3 nm

50 nm

[001][110]

(a)

Fig. 1. (a) Large-scale STM image of the clean Pd(110) substrate showingsteps, which run mainly along the ½1�10� direction. Tunneling parameters:UBias = 1.3 V, IT = 80 pA. (b) Atomically resolved STM image of theclean Pd(110) surface. Tunneling parameters: UBias = �7 mV, IT =150 pA.

4114 M. Kralj et al. / Surface Science 600 (2006) 4113–4118

or He-atom scattering [7–10]. The collected data reveal newinsights in the early stages of the restructuring and thekinetics of the reconstruction process.

2. Experimental

The experiments were conducted in an ultrahigh vacuum(UHV) system with a base pressure in the low 10�9 Parange that features a home-build STM. The microscope’sscanning head was constructed according to the well-known design of Stipe et al. [11]. The STM data wereanalyzed using the WSxM� freeware image processingsoftware [12]. The UHV system is also equipped with aquadrupole mass spectrometer (QMS) and Auger electronspectroscopy (AES). The sample cleaning procedure in-cluded repetitive cycles of argon-ion sputtering (15 min),annealing at 700–750 K in 1 · 10�5 Pa of oxygen (20–25 min), oxidation at room temperature (5 · 10�5 Pa,5 min), and final annealing in UHV up to 1100–1200 K(10 min). Since the AES signals of Pd and C overlap,AES is not very sensitive to carbon contaminations. There-fore, in order to check the cleanliness of the surface, we fol-lowed the desorption of reactively formed CO during thefinal annealing in UHV. We stopped the cleaning cycleswhen no observable quantities of CO could be detectedanymore. The quality of the surface was then immediatelychecked with STM. This preparation procedure leads to asurface of much higher quality than procedures that justconsisted of sputtering and annealing cycles that were alsoreported in the literature for Pd(11 0) [13]. The stability ofthe experimental setup allowed us to expose the surface to adesired hydrogen pressure, even during STM imaging. Inthis way we were able to follow the very initial hydrogen-induced structural changes at specific positions on the sam-ple surface. For calculated exposures we did not take intoaccount the relative sensitivity factor of H2 for the coldcathode gauge used for the pressure measurement. All datawere acquired at room temperature.

3. Results and discussion

A large-scale STM image of clean Pd(110) is shown inFig. 1(a). For the well-prepared surface we regularly foundregions with terrace widths of 100–200 nm. Although thesteps do not have a preferential orientation on thePd(110) surface it is quite easy to identify their crystallo-graphic orientation because steps that do not run alongthe close-packed ½1�10� direction are rough [14]. InFig. 1(a), the steps propagate almost in the close-packeddirection, which is indicated by the smooth edge appear-ance. The measured step height of 1.45 ± 0.05 A is in goodagreement with the actual value for the interlayer spacing

perpendicular to the Pd (110) surface of 1.375 A. The rect-angular unit cell of the Pd(110) lattice can be identified inthe atomically resolved STM image in Fig. 1(b). This imagewas obtained approximately 2–3 h after the last prepara-tion step and a number of single atom wide missing rows

(dark) can be seen. Occasionally narrow added rows(bright) are also observed. Considering the results ofKampshoff et al. [5] we argue that the surface restructuringis locally driven by the adsorption of a very small amountof residual hydrogen. Supposing a maximal hydrogenresidual pressure of p(H2) � 5 · 10�9 Pa we estimate thatthis surface was in total exposed to not more than 0.4Langmuir (L) of hydrogen.

This observation triggered a more detailed investigationof the initial surface restructuring processes through con-trolled dosing of small amounts of hydrogen. The resultof such an experiment is displayed in Fig. 2. The overallobservation time was 240 min, which corresponds to anaccumulated dose of about 7.6 L (p(H2) � 7 · 10�8 Pa).The STM images were taken at intervals of 30 min. Sinceeven atomically resolved STM images do not reveal theposition of individual hydrogen atoms, we have used a sta-tistical analysis based on the measured lengths of dark andbright rows, respectively, to characterize the reconstruction

(d)

(a)

60 min

(b)

120 min

(c)

240 min

Fig. 2. (a)–(c) STM images obtained during hydrogen exposition(p(H2) = 7 · 10�8 Pa). (d) STM image taken at the same surface position,after dosing conditions were changed (see text for details). Image sizes:81 · 81 nm2; tunneling parameters: UBias = 44 mV, IT = 120 pA.

0 50 100 150 200 250

0

200

400

600

800

Dosing time [min.]

Len

gth

[nm

]L

engt

h ra

tio

p(H ) = 7·10 Pa2-8

- dark lines

- bright lines

(a)

(b)

0 2 4 6 8

0 2 4 6 8

2

3

4

5

6

(c)

0.00

0.01

0.02

0.03

0.04

0.05

0.06

0.07

Quantity of supplied H2 molecules [ML]R

estr

uctu

red

frac

tion

of th

e su

rfac

e [M

L]

H2 exposure[L]

H2 exposure[L]

0 2 4 6 8 10 12

Fig. 3. (a) The length of dark and bright stripes as a function of hydrogendosing time. (b) The ratio of the length of dark lines to the length of brightlines as a function of exposure. (c) The increase of the restructured fractionof the surface, A, as a function of hydrogen exposure. The full linecorresponds to an exponential fit.

M. Kralj et al. / Surface Science 600 (2006) 4113–4118 4115

process. It is clearly seen from Fig. 1(b) that the darkgrooves are one atom wide. Furthermore, our data supportthe interpretation that the added bright rows consist of Pddimer-chains [5]. This notion holds, however, only for theinitial stages of the restructuring and only in the middleof large terraces. Step edges behave quite differently aswe will demonstrate below. Three STM images shown inFig. 2(a)–(c) correspond to dosing times of 60, 120, and240 min at p(H2) = 7 · 10�8 Pa. The restructuring processis, of course, more rapid for higher hydrogen pressure. Thiscan be seen in the example of the STM image in Fig. 2(d),which shows the same place as the previous images but wasobtained 10 min after the dosing pressure was increased to5 · 10�5 Pa, which corresponds to an additional 225 L.

The results of the statistical analysis of the STM imagesin Fig. 2(a)–(c) are shown in Fig. 3. The measured lengthsof dark and bright rows are displayed as a function of thedosing time and are indicated with dark circles and greysquares (Fig. 3(a)), respectively. Interestingly, the calcu-lated ratio of the two lengths is always larger than two(Fig. 3(b)) but roughly constant. This simply accounts forthe fact that initially on the terraces significantly more darklines than bright ones are present (Figs. 1(b) and 2(a) and(b)). This suggests that the missing rows are more easilyformed than added dimer rows. From the well knownanisotropic potential landscape of the Pd(110) surface[15] it is quite logical to suppose that the Pd atoms are pre-dominantly mobile in the ½1�10� direction and that the atomtransport along the [001] direction can be neglected. Undersuch circumstances the fast moving single atoms that wereexpelled from the surface should meet and create dimers as

building blocks of bright dimer rows. Obviously, the crea-tion of stable added dimer rows is initially suppressed be-cause the expelled atoms rapidly migrate towards thesteps, where they are trapped.

A detailed analysis of the kinetics of the terrace restruc-turing leads to an exponential increase of the restructuredfraction of the surface as a function of hydrogen exposure(Fig. 3(c)). This behavior suggests an autocatalytic process[16] in which the restructured surface area plays an activerole in the hydrogen adsorption process. The kinetics ofthe hydrogen-induced reconstruction observed here differsfrom the linear law proposed in Fig. 2 of Ref. [5]. However,

(a)

15 nm

Displacement [nm]

STM

hei

ght [

Å]

(b)

1

0

1

0

20151050

[001]

[110 ]

Fig. 4. (a) STM image of a rough step obtained after 25 L of hydrogenwere dosed at p(H2) = 1 · 10�8 Pa. Tunneling parameters: UBias = 1.27 V,IT = 120 pA. (b) Line profiles taken at the positions indicated in (a).

4116 M. Kralj et al. / Surface Science 600 (2006) 4113–4118

if one takes a closer look at the data presented by Kamps-hoff et al., the typical S-shaped behavior as it is expectedfor an autocatalytic reaction is visible in their data [5].The full understanding of the adsorption, dissociationand sticking processes for a molecule on a metallic surfaceis difficult to achieve. For the hydrogen on transition metalsurfaces the diffusion and adsorption of the atomic hydro-gen into subsurface sites makes this understanding evenmore difficult. It is also well known that the molecularand atomic sticking coefficients can have quite different val-ues [17]. In spite of these complications we believe thatfrom our data, taking into account the simple assumptionsdescribed below, it is possible to obtain some new informa-tion related to the initial sticking of hydrogen on Pd(110).

The STM experiment discloses the observable restruc-turing of the palladium surface, i.e. the creation of missingand added rows due to the displacement of the palladiumatoms. This restructuring creates (111) microfacets andprovides threefold sites. These sites are suitable sites forthe dissociative adsorption of H2. As it was shown in arecent STM study, a dissociative hydrogen adsorption onPd(111) requires aggregates of at least three availablethreefold sites [18]. In line with the calculations forPd(110) [2,3] we will assume that the observed restructur-ing includes the final stabilization of missing and addedrows due to the adsorption of single hydrogen atoms inthe threefold sites. Thus, the quantity of the adsorbedhydrogen scales directly with the restructured fraction ofthe surface. Comparing the provided H2 dose and therestructured surface area we obtain the following func-tional relation (full line in Fig. 3(c)):

AðxÞ ¼ 0:0039 � expx

4:03

� �� 0:0015 ð1Þ

where x refers to the quantity of supplied hydrogen mole-cules (in ML) and A(x) is the restructured fraction of thesurface (also in ML). The simplifications made for the ob-tained expression are that (a) the idealistic Hertz-Knudsenformula for the hydrogen incident flux was used, (b) thequantity of supplied hydrogen molecules was calculatedin terms of monolayers with respect to the atom densityof the unreconstructed Pd(110) surface, and (c) the relativesensitivity factor of H2 for the cold cathode gauge was nottaken into account for the pressure measurement. The off-set of 0.0015 ML reflects the fact that a small part of theterrace was already restructured at the beginning of thecontrolled hydrogen dosing due to the adsorption of resid-ual hydrogen prior to the systematic dosing experiment.From the derivative of Eq. (1) the sticking coefficient forH2 can be calculated. In the range from 1–8 L it rangesfrom 0.001 to 0.02. The estimated values agree very wellwith the consideration of the dissociative H2 sticking prob-ability on Pd(111) where a value in the range of 0.005–0.008 was proposed [18]. Please note that the experimentin Ref. [18] was performed at 37–65 K, whereas our mea-surements were done at room temperature. Our data agreealso well with the experimental results of Dean et al. [19].

We also like to emphasize that our values in Fig. 3 referonly to observations in the middle of large terraces, i.e. freeof the influence of steps.

We now turn to restructuring of the steps running in the[00 1] direction. Fig. 4(a) shows the STM image of suchstep that was obtained after a total hydrogen dose of25 L at p(H2) = 1 · 10�6 Pa. It is obvious that the presenceof the step has a strong impact on the evolution of brightlines on the lower terrace and of dark lines on the upperterrace. As we showed earlier, the bright lines created onthe terraces consist of dimer-lines. However, as seen fromFig. 4(a), in the vicinity of the step edge single atom rowscan be found. In this particular case some of them arelonger than 50 nm, which is consistent with the initiallydominant migration of single Pd atoms along the ½1�10�direction. Moreover, the line profile shown in Fig. 4(b)indicates that the dominant distance between two lines is0.78 nm, i.e. twice the palladium atomic periodicity in the[00 1] direction. This immediately corresponds to the crea-tion of compact nano-sized regions of the (1 · 2) missing-row reconstruction, which is the most stable configurationfor the adsorption of hydrogen on Pd(110) [3]. The obvi-ously enhanced concentration of the described structuresin the step edge regions can be compared to observationsof the oxygen-induced restructuring of the same surface[20]. Evidently, rough steps play an important role in the

M. Kralj et al. / Surface Science 600 (2006) 4113–4118 4117

reconstruction of the Pd(110) surface. The dynamics ofroom temperature step fluctuations on clean Pd(110) wasa topic of a study described in Ref. [14] and it was shownthat step movements take place on the time scale of min-utes. Although we must note that the observed processesmay have to do with the influence of residual hydrogen,that study corroborates the clear tendency of rough stepstowards a more coherent (1 · 2) missing-row restructuring.

All structural motives that have been discussed so farare present in the STM image shown in Fig. 5. Accord-ingly, we classify these motives as being of type A (missingrows), B (single atom wide added rows), C (dimer addedrows), and D (added row (1 · 2) reconstructions), whichare also schematically shown in Fig. 5. Concerning thedimer rows we found a peculiarity, which has not beenobserved before. Along these rows a change in width andapparent height is usually observed, which is evidenced inthe two line profiles shown in Fig. 5. By comparing theapparent height of the narrower section of the dimer rowswith single rows (�1 A in both cases) we conclude that thedimer rows (type C) frequently terminate in monomer rows(type B). This explains the asymmetric appearance of thedimer rows. We must also mention that for the particularimaging conditions applied during the acquisition of theSTM image in Fig. 5 (UBias = 44 mV, IT = 80 pA) theapparent height of features type A, B and D is largelydifferent even though they should in principal be equal tothe step height of the Pd(110) surface. Only the dimer rows

Displacement [nm]

STM

hei

ght [

Å] 1.5

1.0

0.5

20100

B C

D

A

B1.5

1.0

0.5

0.0

-0.5

3020100

C

10 nm

A

C

B DB

Fig. 5. STM image obtained after a hydrogen exposure of 20 L(p(H2) = 7 · 10�7 Pa) that illustrates reconstruction motives observed inthe initial stages of restructuring. Tunneling parameters: UBias = 44 mV,IT = 80 pA. Two profile scans shown below the STM image relate to thewhite and black dashed line. Model of the surface showing the differentstructural motives is also shown.

(type C) show the correct height of �1.4 A. The missingrows (type A) are imaged as grooves of only 0.5–0.6 Adepth. The single added rows (type B) and the added(1 · 2) reconstructions (type D) possess an apparent heightof �1 A. This suggests that under these imaging conditionselectronic structure effects are responsible for the differentheight of the features. This is corroborated by the fact thatin the STM image in Fig. 4 the single added rows are

imaged with the correct height (�1.4 A) when using differ-ent tunneling parameters (UBias = 1.27 V, IT = 120 pA).Conversely, in Figs. 4 and 5 the missing rows (type A)are imaged with approximately the same depth. We inter-pret this in terms of a tip convolution effect; the tip is forgeometrical reasons not able to sink into the grooves to im-age the correct depth.

4. Conclusions

In conclusion, we have studied with STM the initialhydrogen-driven restructuring of the Pd(110) surface.The hydrogen-induced reconstruction of the Pd(110) pro-ceeds via an autocatalytic process. The initial sticking coef-ficient for H2 on a Pd(110) terrace is �0.001. Thestructural motives, which can be found on large terracesand steps in the [001] direction differ significantly. On theone hand, in the middle of the terraces initially vacancy-rows and dimer-lines are created. The rough steps, on theother hand, represent the ideal front for the restructuringsince the morphology of the upper and lower terracechanges directly towards an ideally (1 · 2) missing-rowreconstructed surface. Since the terraces have a finite size,restructuring motives of both regions intermix in the laterstages of surface reconstruction.

Acknowledgements

M.K. wishes to thank the Alexander von Humboldtfoundation for the financial support.

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