Hydrogen-induced reconstruction of transition metal surfaces

Progress in Surface Science, Vol. 42, pp. 245-255 0079-6816/93 $24.00 + .00 Printed in the U.S.A. All rights reserved. Copyright © 1993 Pergamon Press Ltd.

HYDROGEN-INDUCED RECONSTRUCTION OF TRANSITION METAL SURFACES

K L A U S M O L L E R

Lehrstuhl for Festk(~rperphysik, Universit#t Erlangen-NOmberg Staudstr. 7, D-8520 Erlangen, Germany

ABSTRACT

This study compares the results of a number of recent papers on hydrogen adsorption on Rh(110), Rh(311) and Fe(211) as well as on Ni(111) and Fe(110) surfaces. I t part icular ly deals with the structural aspect of these low energy electron di f f ract ion (LEED) investigations and correlates them, i f available, with respective thermodesorption data. Upon dissociative adsorption by a non activated process hydrogen induces local displacements of the atoms about the adsorption sites. With increasing coverage these displacements order to form a sequence of weakly reconstructed phases and gradually l i f t the surface layer relaxation of the formerly clean surface. Along close packed rows of metal surface atoms hydrogen atoms tend to occupy threefold coordinated adsorption sites which, in turn, arrange in single or double chains. The coverage dependent per iodic i ty of these adlayer structure elements together with the respective sh i f t buckling of the substrate surface generates the observed superstructures. Since not only open but also close packed surfaces show this weak (and sometimes strong) reconstruction upon hydrogen adsorption i t should be generally considered in a11 adsorption systems.

CONTENTS

1. INTRODUCTION

2. GENERAL EXPERIMENTAL REMARKS

3. RECOLLECTION OF RESULTS

4. DISCUSSION

REFERENCES

245

246 K. MOiler

I. I NTRODUCTI ON

Hydrogen adsorption is being investigated by many studies of fundamental

research as well as in connection with important applications. The f ie ld

is already carefu l ly reviewed, and the excellent survey by Christmann [1,2]

also includes a comprehensive l i s t of hydrogen structures observed on metal

surfaces. This paper is to draw attention to a feature of hydrogen

adsorption which has not yet systematically been studied and therefore is

mostly ignored: The influence that hydrogen as an adsorbate exerts on i t s

substrate's local and long range order. Although the phenomenon of

adsorbate induced reconstruction is widely accepted, l imited experience

exists on hydrogen induced reconstruction. This is so for an obvious

reason: Most of our kowledge on surface structures comes from LEED

experiments and LEED structure analyses. Since hydrogen is a weak scatterer

compared to the substrate atoms, superstructure spots due to hydrogen

adsorption are mostly also weak and therefore d i f f i c u l t to measure. With

a few exceptions i t was therefore generally assumed that these weak spots

are generated by the adsorbate alone and that hydrogen has no impact on the

substrate. I ts surface structure was considered to be r ig id supplying a net

of f ixed adsorption sites. With this assumption several systems have been

investigated (H on N i ( l l I ) [3], Pd(111) [4], Ru(O001) [5] , Fe(110) [6],

Pd(llO) [7], Ni( l lO) [8] ) .

Recent progress in LEED structure analysis which includes experimental

as well as theoretical improvements with respect to sens i t i v i t y and

accuracy has led to new investigations. As a f i r s t general resul t i t turned

out that the in tens i t ies of hydrogen induced superstructure spots averaged

over a certain energy range are not as low as i~o of the respective

in tens i ty average of integer order spots, as expected. Instead, the rat io

sometimes exceeds 54. Moreover, these extra spots are v is ib le up to high

primary energies in contrast to the energy dependence of the scattering

power of hydrogen. This c lear ly shows that the substrate contributes to the

extra spot in tens i t ies and hence must be reconstructed. Consequently surface

atom coordinates must be treated as structure parameters to be determined

by the LEED structure analysis. For H/Rh(IIO) a new set of structure data

is available and less complete results have been obtained for Rh(311) and

Fe(211). The structural properties of these systems wil l be compared with

one another and with data obtained from TDS experiments.

Hydrogen-Induced Reconstruction of Metals 247

2. GENERAL EXPERINENTAL REMARKS

Working with hydrogen makes i t necessary to operate the vacuum chamber at

a background pressure well below 10 "I° mbar. Adsorption experiments were

performed at hydrogen pressures of about 10 .9 mbar and at temperatures of

the specimen between 40-300 K. All LEED data were taken by the computer-

controlled video system AUTOLEED [9-11] which a11ows fast and reliable

measurements of spot intensities and spot profiles as a function of any

external parameter desired. In order to accurately measure even very weak

features in the diffraction pattern an image intensifier camera was used.

The system was equipped with a quadrupole mass spectrometer in a differen-

t i a l l y pumped arrangement to measure thermal desorption spectra. Relative

hydrogen coverage for different adsorption phases can be obtained by

integration of the corresponding thermal desorption curves. To get absolute

coverage data we use the well determined structure of one of the ordered

phases as a coverage reference. For more specific experimental details such

as sample preparation or data handling see the respective original papers

cited in the following section.

Rh (110) Rh (311) Fe (211)

Fig. I. Surface models

248 K. MOiler

3. RECOLLECTION OF RESULTS

The data to be discussed were taken at those surfaces, whose structure

models are displayed in Fig. I: Rh(110), Rh(311) and Fe(2]1). They are

re la t ive ly open surfaces, but all three contain close packed rows of atoms.

I t was suggested already before these studies that hydrogen wi l l adsorb in

a si te of high coordination, possibly a threefold coordinated site. Rh(311)

offers both threefold and fourfold coordinated adsorption sites.

Rh(110): The clean surface displays a Ix i LEED pattern, and the structure

analysis reveals multilayer relaxation with a f i r s t layer contraction of

about 7~ [12]. Hydrogen adsorption reduces this relaxation, and i t is

remarkable that i t even l i f t s i t at fu l l coverage. With increasing coverage

hydrogen leads to f ive different ordered phases as seen in the LEED pattern

and f i r s t published by Christmann et al [13]" lx3-H, lx2-H, 1x3-2H, 1x2-2H

and 1x1-2H. The phase diagram (Fig. 2) shows their s tab i l i t y ranges as a

220-

200- v 180- , B

160 I . . 140

,_° 120- 100- E 80- 60-

~0-

ff/ ~1[C I

o . I

I != ~ °

-I- - - I - - i i i i

, , I J i j 0.2

" l - t ~

i t ~

x

÷ " r r ~

I i i i I

- - ~ ,

J I I I I I

0.4 0.6 0.8 1.0 1.2 1.4 coverage

H/Rh(110)

- r - r

X X

I I I I i

1.6 1.8 2.0

Fig. 2. Phase diagram for H/Rh(110), from [12]


function of coverage and temperature. I t was developed by beam prof i le and

integral spot intensi ty measurements [12]. The commensurate phases are

separated by coexistence regions indicating a f i r s t order transi t ion between

them. A number of detailed LEED structure analyses all cited in [121

determined the adsorption geometry of these structure phases. The result

is that, indeed, the hydrogen atoms occupy sites of nearly threefold

symmetry which are arranged in l inear chains along the close packed rows

of substrate atoms. Consecutive structures at low coverage only d i f fer in

the distance of H-covered rows. But as soon as (~ exceeds 0,5, double rows

are being formed, f i r s t at the larger distance of three times the substrate

per iodic i ty length, then at two times that length. Finally, at fu l l

coverage of (~=2, every row of substrate atoms is decorated with a double

row of hydrogen atoms. Only two (out of five) of these real space

configurations are shown in Fig. 3. This development of phases correlates

H/Rh(311) H/Rh (110) H/Fe(211)

lx3-H l x 3 - H c(2x6)-8H

ell= 1/3 e = 0. 33 8H= 2/3

1x3 -2H 1 x 3 - 2H c( 2.6).-16H ell=2/3 e=0.66

0H=4/3

Fig. 3. Real space con f i gu ra t i on models f o r se lec ted ordered phases. Ful l dots mark adsorpt ion s i t es .

250 K. MOiler

• '~,,,4

¢ ' q

/ ~ I-I/Rh(llO) Tad: 90K

0.83L

t// ~ O.17L

/A/~ 0.65L / / / / / / / ~ -~ ' /~" O'IIL O.05L

100 150 200 250 300 350 400

Temperature in K

Fig. 4. TD spectra for H/Rh(110). Numbers indicate hydrogen exposure at the beginning of each spectrum, from [12]

well with the TD spectra displayed in Fig. 4. While the y state corresponds

to the single row phases, the fl state starts to appear with the formation

of double rows. The a state, f inally, indicates the direct neighbourhood

of H-double rows.

Rhodium atoms decorated with hydrogen are displaced from their former

equilibrium positions. Their exact positions as well as those of the

hydrogen atoms vary with coverage, but the whole restructuring can best be

described as a weak "shif t buckling" reconstruction with an amplitude of

about 0.03-0.05 A. I t is total ly l i f ted at full coverage 0=2.

H/Rh(311): There are many similarities to the previous system of

H/Rh(110). The clean surface shows an even stronger multilayer relaxation,

and a number of commensurate phases can be observed [14] including the Ix3-

H and the lx2-H superstructures with their surface models shown in Fig. 3.

Although a complete structure analysis is not yet available we have reason


to assume the same kind of adsorption site, the formation of linear chains

and double chains in much the same way. From the intensity ratio of extra

to integer order spots we again postulate a weak shift buckling of the

surface. But at coverages exceeding 0=0.7 a strona Ix2 adsorbate induced

reconstruction is observed in contrast to the previous example. Fig. 5

shows a series of TD spectra whose 8, 7 and fl states again can be

correlated with observed commensurate structure phases. The a state,

however, appears together with the strongly reconstructed phase and cannot

be saturated. I t is therefore assumed that the surface at that stage opens

diffusion channels to an area underneath the f i r s t atom layer and hence

supplies sites for subsurface hydrogen. A possible surface [14] model is

shown in Fig. 6. Although the suggestion of subsurface hydrogen cannot yet

be proven i t is supported by the fact that hydrogen uptake in the a state

is not accompanied by a workfunction change which, of course, is observed

for all other phases.

H/Rh(311)

T~d = 85 K

(,. 112L BI-I I I ~ ' ~ ' ~ "

100 150 200 250 300 350 400

Temperattae in K

Fig, 5, TD spectra for H/Rh(311), from [14]

252 K. MOiler

(o)

Fig. 5. Surface model for Rh(311) a) clean surface, b) lx2 reconstruction. Moved atoms marked by +, from[14]

Fe(211): The third example is also a surface with close packed rows as

displayed in Fig. I. Similar to the previous cases is the formation of a

number of commensurate structure phases with increasing hydrogen coverage

[15]. Surface models for two of these phases are shown in Fig. 3. There is

again the formation of chains of hydrogen atoms. Due to systematic

extinctions in the diffraction pattern of one of the phases, however, a

resulting glide symmetry plane introduces zig-zag-chains in the model

instead of linear chains. But there is also another striking difference:

In contrast to the previous examples the commensurate phases are only

metastable and irreversibly transform to a strongly reconstructed phase

beyond about 200 K, a temperature which depends sl ight ly on coverage. The

stabi l i ty ranges of all phases are displayed in Fig. 7. Although there are

no structure analyses available the reconstructed phase has most probly a


lx2 missing row s t r u c t u r e which can on ly be l i f t e d by desorp t ion of

hydrogen.

(D w.,

3 0 0 -

280 -

260 -

240 - 2 2 0 -

200 -~ 180 z

160 £

\ \

\ (lXZ)s \ \

140 Latt ice [ ::z::

lZO - gas ~ l ~,

60 . . . . . . . . . . . . . . . Y _ i . . . . . .

40

H/Fe(211)

~X . . . . Desorption \ ..I""" ".. ~// boundary

' I ' I ' I ' I ' I ' I ' I ' I ' I '

0 0 0 2 0 4 06 08 l 0 12 14 16 18 2 0

Coverage

Fig. 7. Stab i l i ty ranges of metastable commensurate phases of H/Fe(211) from [15]

4 . D I S C U S S I O N

The adsorption systems described above show all indications for weak (and

strong) hydrogen induced reconstruction. Following dissociative adsorption

hydrogen occupies adsorption sites of high coordination and rearranges i ts

local environment. The di rect ly neighbouring substrate atoms are displaced

in such a way that the next adsorbing hydrogen atom is attracted to a

part icular neighbour site, which was prepared by the local reconstruction

induced by the previous one. Although the displacements are small they

change the local symmetry and predetermine the neighbour adsorption site.

According to the symmetry of the local reconstruction linear, or zig-zag-

chains or, maybe, other elements of adsorbate order can result. I t is,

254 K. MOiler

however, not the symmetry of the substrate surface that determines the

details of the local displacements. Hydrogen forms different types of

chains on rather similar surfaces such as Rh(110) and Fe(211).

But the interaction of hydrogen with i ts substrate can lead to even

larger displacements of the substrate atoms and hence to strong recon-

struction. In case of Rh(311) this comes about just by higher coverage, for

Fe(211) additional thermal activation is necessary. In other examples such

as W(IO0) [16 and references therein] adsorbed hydrogen is known to alter

the clean surface reconstruction (symmetry switch).

(2x2)-2H/Ni(111) (2x2) -2H / Fe (110)

:R

Fig. 8. Low temperature 2x2-2H adsorption phases. Full dots indicate hydrogen adsorption sites.

All the surfaces discussed so far resemble more or less open structures,

and i t is interesting to ask i f close packed surfaces also show the weak

reconstruction upon hydrogen adsorption. For this purpose the low

temperature 2x2-2H phases on Ni(111) [17] and on the nearly close packed

Fe(110) surface [18] have been studied. As a result, Fig. 8 shows again the

threefold coordinated adsorption sites. But more importantly, the detailed

LEED structure analysis reveals a definite buckling of f i r s t and second

]ayer substrate atoms! Even more interesting, however, is the finding that


the t r i p l e of Ni at,rosa associated with a hydrogen atom is l i f t e d while

those atoms not covered by hydrogen are pushed inwards. And i t is just the

other way around at the Fe surface in accordance with the d i f ferent sign

in workfunction change upon hydrogen adsorption on Ni(111) and Fe(110).

Although only l imited structure data are available so far i t can be

concluded from the above discussion that even close packed metal surfaces

must be considered "soft" with respect to hydrogen adsorption. At least

weak reconstructions with displacements of the substrate atomsa of the order

of a few hundredths of an Angstrom occur. These apparently determine the

neighbouring adsorption sites and may even extend to long range order.

.

2. 3.

4.

5. 6.

7.

8.

9.

10.

11. 12.

13.

14.

15.

16.

17.

18.

REFERENCES

K. Christmann, Surf. Sci. Rep., g, 1 (1988). K. Christmann, Molecular Physics, 66, 1 (1989). K. Christmann, R.J. Behm, G. Er t l , M.A. Van Hove and W.H. Weinberg, J. ChenL Phys., 70, 4168 (1979). T.E. Felter, E.C. Sowa and M.A. Van Hove, Phys. Rev. B, 40, 891 (1989). M. Lindroos, H. PfniJr and D. Menzel, Surf. Sci., 192, 421 (1987). W. Moritz, R. I mbihl, R.J. Behm, G. Ertl and T. Matsushima, J. ChenL Phys., 83, 1959 (1985). M. Skottke, R.J. Behm, G. Ert l , V. Penka and W. Moritz, J. Chem. Phys., 87, 6191 (1987). W. Reimer, V. Penka, M. Skottke, R.J. Behm, G. Ertl and W. Moritz, Surf. Sci., 186, 45 (1987). K. Miil le r and K. Heinz, in: The Structure of Surfaces, Vol. 2 of Springer Series in Surface Science, ed. by S.Y. Tong and M.A. Van Hove (Springer Berl in), 105 (1986). P. Heilmann, E. Lang, K. Heinz and K. MEiller, in: Determination of Surface Structure by LEED, ed. by P.M. Marcus and F. Jona (Plenaum, New York), 463 (1985). K. Heinz, Progr. Surf. Sci., 27, 239 (1988). Nicht1-Pecher, J. Gossmann, W. Stammler, G. Besold, L. Hammer, K. Heinz and K. Mi]ller, Surf. Sci., 249, 61 (1991). K. Christmann, M. Ehsasi, J.H. Block and W. Hi rschwald, Che~ Phys. Let t . , 131, 192 (1986). W. Nichtl-Pecher, W. Stammler, K. Heinz and K. MEiller, Phys. Rev. B, 43, 6946 (1991). R. Schmiedl, W. Nichtl-Pecher, K. Heinz and K. MLiller, Surf. Sci., 235, 186 (1990). G. Schmidt, H. Zagel, H. Landskron, K. Heinz, K. MUller and J.B. Pendry, Surf. Sci., 271, 416 (1992). H. Landskron, H. Fricke, L. Hammer, K. Heinz and K. M(iller, to be publ i shed. W. Nichtl-Pecher, J. Gossmann, L. Hammer, K. Heinz and K. MLiJler, J. Vac. Sci. Techno]., /L10(3), 501 (1992).

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Hydrogen-induced reconstruction of transition metal surfaces