Ag
ffiEl
ri)rfil
Available
onlineat www.sciencedirect.com
A
acrENce
Rrcro
(cl)ot
Y.Lz
ET^SEVIER
SURFACE SCIENCE
SurfaceScience
530(2003)37-54
www.clscvier.com/locate/susc
Adsorptionof NH3 on oxygenpre-treated
Ni(l I 1)
*,
E . L a kso n o ,A . Galtayr ieS C. Ar gile, P. M ar cus
Luboratoire da Pb,sico-Chirniedcs Surfuces CNRS (UMR 7045), Uniuarsiti Piarra ct Muria C'urLc,
Emlc Nutktnulc Suptricurc da Chitnic dc Puris, ll rua Piarrc ct Muric Ctrria. l--75005 Puri.t, l"runtt
R c c c i v e d l 4 N o v e m b e r 2 0 0 2 ; a c c c p t c df o r p u b l i c a t i o n 2 5 J a n u a r y 2 0 0 i
Abstract
T h ea d s o r p t i oonf N H 3 o n o x y g e up r e - t r e a t eNdi ( l I l ) s u r l a c e sh a sb e e ns t u d i e calt r o o m t e m p e r a t u rues i n gX - r a y
(XPS).Oxygenpre-treatments
photoelectron
havebeenperformedat 650 K. This protocolleadsto a
spectroscrpy
phase+ NiO islands)
"two-phase
domain"(O-adsorbed
The investigation
overa largerangeof oxygenexposures.
of the
phaseis present;the
surfacereactivitytowardsNH3 showsthat ammoniais adsorbedprovidedthat the O-adsorbed
phasecovcrage.
with the O-adsorbcd
Two N-adspccics
surfacereactivityincreases
havcbccudctectcdfrom the N ls
corelevelpeaksat 399.8+0.2and 397.8f0.2 eV and assigned
to molecularNH3 and dissociated
NH2 species,
reThe molecularadsorptior.r
resultsfrom directimpingement
whereasthe dissociated
spectively.
of the NH: molecules,
oneresultsfrom thedissociation
ola part of thepreadsorbed
molecular
At saturation,
thedissociated
species.
species
is
(>0) of the surfaceby the
the more abundantone (about4/5 of the total N ls peak)whateverthe initial coverage
pl'rase.
The XPS data indicatethat this dissociation
is correlatedto the formationof OH from oxygenof
O-adsorbed
phaseand hydrogenabstraction
phaseis absenton
the adsorbed
from themolecularammonia.Whenthe O-adsorbed
the surface,i.e. for cleanNi(l I l) or the completeNiO layer,noneof thesesurfacereactionswith ammoniaoccurs,
underthe sameadsorptionconditions.
@ 2003ElsevierScience
B.V. All rightsreserved.
Keywords: Chemisorption; Anrmouia; Low indcx singlc crystll surlaccs;Nickel; Nickcl oxidcst Oxygcn; X-ray photoelectron
spectroscopy
l, Introduction
Over the past three decades,the adsorptionand
decompositionof ammonia on metals,as well as on
metal oxide layers, has attracted much interest.
Structural techniques as well as electronic and
'Corresponding
a u t h o r . T e l . : + 3 3 - l - 4 1 2 ' 7 6 ' 7 3 7f a; x : + 3 3 - l 46340'15t.
jussicu.fr (A. GalE-mail address: anouk-girltayrics(r)enscp
tayries).
v i b r a t i o n a ls p e c t l o s c o p i ehsa v e b c e n u s e d .S t u d i e s
of this systemcover model surfaces(singlecrystals
of metals, or metal oxide laycrs on metal single
crystals)as well as more complex ones(polycrystalline metals, oxide layers on metaliic polycrystals),
o n w h i c h m o l e c u l a ro r d i s s o c i a t i v ea d s o r p t i o no f
ammonia can occur over a wide range of temperatures. In all cases,understandingof the mechanisms of ammonia interaction with the surfacesis
essential.There are important applicationsin more
complex processes,for example in heterogeneous
catalysis,where ammonia is either a reactant (for
- seefront matterO 2003Elsevier
Science
B.V. All riehtsreserved
0039-6028/03/$
doi:I 0.I 0 I 6/50039-6028(03)00267-X
ffifriltF#*o*no
.i'
wl
F. l
G
I
2
.9 YO,:l-i.;:.::-
DFHAilii""'.,'.
t*
Erv&
E. Luksono at ul. I SurJutc Scicncc530 (2003) 37 51
examplethe industrial synthesisof HNO3 by ammonia oxidation over a platinum catalyst-known
as "Ostwald proces5"-sr the HCN synthesis,by
reaction of methane with ammonia and oxygen
over a platinum-rhodium catalyst-known as
"Andrussow process") or a product (for example
the industrial synthesisof NH3 from nitrogen and
hydrogen over an iron catalyst-known as "Haber-Bosch proces5"-sr the undesirableproduction of ammonia in the automotive exhaust gas
converter)or even the catalyst (NH3-catalysedsequential surfacereactions[1]). Another exampleis
the corrosion of metals in the presenceof atmospheric pollutants such as ammonia. Surface nitridation for semiconductorapplicationsmay also
be mentioned 12-71.Finally, NH3 is an often used
probe moleculefor investigatingthe acidity of solid
surfaces[8].
The studiesof the chemisorptionof ammonia on
metal surfacespublished up to the early eighties
(1984)have been reviewedby Lambert and Bridge
[9]. The major part of the surfacestudiesof NH:
adsorptionin the eightieswere focusedon the welldefinedsurfacesof transition or noble metals [0l5], at low temperature of adsorption between
about 100 and 200 K. The question of non-dissociative versus dissociativeadsorption behaviour,
and the structure of the surface ammonia complexeswere investigatedby differentelectronicand
vibrational spectroscopiesas well as by thermal
As we will not focus our
desorptionmeasurements.
work on the NH3 adsorption on metals, we only
briefly summarise here that, depending on the
metal, the NH3 ability to dissociateis strongly
decreasingfrom the left hand side to the right hand
side of the transition metal series.A more complete picture of the NH3 adsorption on vanous
metals at the end of this period rvas given by
T h o r n b u r g a n d M a d i x | 6 1 i n 1 9 8 9 .F r o m e x p e r i mental work, it has been shown that in the molecuiar state, ammonia moleculesbond with their
nitrogen end nearestto the surface,and their C3"
symmetryaxis normal to the surface.This bonding
has been interpreted as irnplying that nitrogen
donatesits lone pair of electronsto tl.resurfacc to
form the bond to the metal. However, accordingto
theoretical work of the time (late eighties), the
bonding is mostly electrostaticin nature, involving
the permanentdipole of ammonia and an induced
dipole in the surface.It was statedin this work [ 6]
that the binding site of ammonia was not unambiguously establishedfor any of the quoted single
crystal surfaces,and different electropositivesites
have been proposed implying both models of
b o n d i n g : e l e c t r o nd o n a t i o n o f t h e n i t r o g e n l o n e
pair to the metal as well as electrostaticbonding
on the surface. Recently, an interesting bibliographic review of the previous researchon NH3
adsorption on Ni(l I 0) has been published by
Chrysostomou et al. [7]. The authors also raised
the problem of the stability of molecularlyadsorbedammonia when probing with photons or
electrons,as carly suggestionson the decomposition pathways of ammonia on Ni(l I 0) have been
later questioned (see in Ref. [7]). In particular,
Iaegeret al. [18] have provided clear evidencefor
the occurrence of photon-induced reduction of
a m m o n i a o n N i ( 1 l 0 ) d u r i n g s u r f a c ec h a r a c t e r i sation. Nevertheless,surface analysis techniques
employing eiectron irradiation (LEED, Auger,
H R E E L S . . . ) a n d t e m p e r a t u r e - p r o g r a m m ed e sorption (TPD or TD) have been found to be even
more damaging to the integrity of the adsorbed
ammonia than those based on photon irradiation. All logical dehydrogenated intermediates
expected to form during the surface decomposition of ammonia: NH2(ads), NH(ads), N(ads)
have been proposed on Ni(l I 0). However, there
is some disagreementin the literature on the exact mechanism of ammonia decomposition and
on the intermediatesinvolved in the decomposition.
Despite the large number of works carried out
during the eighties,severalfundamental questions
are still debated in the more recent surface science
literature on the ammonia interactionwith metals:
t h e i n t e r a c t i o n so f a m m o n i a w i t h R h ( l I 1 ) [ 9 ]
have been studied at low pressurewhereasthe attention was previously focused on the decompos i t i o n o f a m m o n i ai n m o d e r a t ep r e s s u r ec o n d i t i o n s
o n l y ; t h e i n t e r a c t i o n so f N H r w i t h R u ( O 0 1 ) [ 2 0 ] ,
Pt(l I l) [21,22), thc multilayer coverage on
Ag(l I l) 1231,the intcractior.rof ammor.riawitl.r
nickel single crystalsstudied by more recent techniques as angle-resolvedphotoemissionextended
fine-structurespectroscopy(ARPEFS) on Ni(l 0 0)
E Luksonoet ul. I SurfaceScience530 (2003)37-54
Ni(l I l) surfaces,as a function of the initial oxygen exposure,investigatedby XPS.
2. Experimental
For the XPS characterisations,Ni 2p, O 1s and
N ls core level spectra have been recorded with
a VG ESCALAB Mk II X-ray photoelectron
spectrometer,with an un-monochromatisedAlK"
anode, at a pass energy of 20 eV. The binding
energieswere referencedto the Ni 2p372line and
A u 4 f 7 p ,s e ta t 8 5 2 . 8a n d 8 4 . 0e V , r e s p e c t i v e l ya,n d
given with an accuracy of 0.1 eV for intense
spectra and 0,2 eV for less intense signals corrcsponding to low coverageadspecies(N ls, O 1s).
Clean and O-treated surfaceswere also checked
from contamination by recording a survey spectrum and the C ls and S 2p core level spectra.The
spectraof the surfacesprior to ammonia adsorption were characterisedat electron take-off angles
of 90o with respect to the sample surface. Connected to the UHV analysis chamber (base pressure 3 x l0-r0 Torr) of the spectrometeris a UHV
preparation chamber (base pressure 5 x l0-r0
Torr) with heating and gas introduction facilities.
The Ni(l I l) sur'faccwas clcancd by cyclcsof Ar+
sputtering and annealing,first in H1 and then rn
v a c u u m .T h i s s e q u e n c ew a s l o u n d t o b c t h e b e s t
compromise,in our experiments,to avoid both S
segregationdue to prolonged annealing under
v a c u u m a n d o x y g e no r c a r b o n r e - c o n t a m i n a t i o n .
After cleaningthe surface,high purity oxygen and
ammonia (from Air Liquide) were introdubed in
the treatment chamber. Oxygen exposureswere
typicallyperformedat Po.:1 x 10-6 mbar (1 x
l0-7 mbar for the two lower 02 exposures).The Opre-treatedNi(l I l) sampleshave beenexposedto
ammonia at room temperature,at I x l0-7 mbar.
Ammonia was typically admittcd for periodb of 3
min in the preparation chamber, after which the
gas was evacuated and the sample analysed by
XPS in the analysis chamber. Exposures in the
preparationchamber arc rcportcd in langmuirs(L)
( l L : l 0 - 6 T o r r ' s ) , N o t e t h a t t h e r e p o r t e dp r e s sures and exposures,in the text or in the figure
captions,are not correctcd for ion gaugc sensitivity. The samplc tempcrature, duling oxygcrl cx-
posures, was measured with a pyrometer. To
analysethe individual contributionsof the Ni 2p:tz,
O ls, and N ls core levels, peak decomposition
was carried out with a computer program using
gaussian/lorentzianpeak shapes, and a Shirley
background.
3. Resultsand discussion
3.1. Oxygen ilteraction on lVi(l I 1) at 650 K prior
to NHj exposlLre
A t r o o m t e m p e r a t u r e ,a n d u p t o 5 0 0 K , i t i s
usually accepted(see,for cxample [40] and refere n c e st h e r e i n ) t h a t 0 3 i r . r t e r a c t i o w
n ith Ni(l I l)
proceedsvia three steps:(i) a rapid chemisorption
ending with a first plateau,(ii) a secondincreasein
oxygen uptake correlatedwith the nucleationand
growth of NiO islandsand then (iii) the lormation
of a full oxide layer, leading to a secondplateau.
Up to the end of the first platcau, it is established
[41] that chemisorptionoccursrvith little effectson
the core level spectra, which indicates that the
surface atoms rctain thcir metallic character. At
room temperature,this chemisorption stage gives
rise to a p(2 x 2) structurc for a maximum covera g e o f l l 4 o f a m o n o l a y e r .A t c o v e r a g e s _ a b o v e
- 0 . 2 8 M L , s o n i ca u t h o l s f c p o f t r r ( V 3 . o / l ) R 3 0 "
structure,and some do not (see,for example,[40]
and referencestherein). Raising the temperature
a b o v e- 3 0 0 K c a u s e st h e ( r , 4 / / 3 ) R 3 0 " s t r u c t u r e
(when observed)to convert to a split p(2 x 2) [a0].
In the domain wherc the NiO formation occurs,
the Ni core level spectra reflect the occurrenceof
the oxide nucleation and passiveoxide film formation. The mcchanisms currently accepted for
the oxidation involve the nucleation and growth
o f o x i d e i s l a n d s w i t h i n t h c O - a d s o r b e dp h a s e .
The growth is cxpcctcdto occur at tl.repcrimeterof
the oxide nuclei with oxygen supplied by surface
diffusion [40,42]. It has been proposed that, up to
473 K [40], the hexagonal, kinetically favoured
NiO(l I l) structurc grows rapidly to coalescencc
c o v e r i n g t h e e n t i r e N i ( l I l ) s u r f a c e .A t h i g h e r
t e m p e r a t u r e( T > 4 1 3 K ) , N i O ( 1 0 0 ) s t a r t st o f o r m
i r r c v c r s i b l y ,i n d i c a t i n gt h a t t h i s i s t h e t h c r m o d y narnically favoured form of thc oxide also on
E. Luksono at ul. I Surfucc Stience 530 (2003) 37-54
hydroxylated NiO islands were the minor component.
As indicated above, in the frequent controversy
betweenOH(a) and O(a) or O- binding energies,
the assignmentof binding energy of the O(a) species remains questionable.We performed another
type of test to check the possibility of an OH
contribution. A clean Ni(l I 1) samplewas treated
in 02 at room temperature,and subsequentlyexposed to water vapour at room temperature.The
O 1s region clearly showeda significantincreaseof
the high binding energy contribution located at
531.5A 0.2 eV that we attributed to the presenceof
hydroxyl groups on the surface.
ln our seriesof experimentsit comes out that.
after oxidation at 650 K, the O 1s core level peaks
presena
t n h y d r o x y lc o n t r i b u t i o na t 5 3 1 . 5f 0 . 2 e V ,
corresponding to about 10ol' of the total O ls
peak area. According to a previou5work [46], this
surface hydroxylation likely corresponds to the
NiO(1 I 1) grains,which are the minor components
of the NiO film at high temperature,as NiO(100)
starts to form irreversiblyabove 473 K [40]. it has
been also noted that for the lower 02 exposures
( c o r r e s p o n d i ntgo v a l u e so f t h e X P S i n t e n s i t yl a t i o
1 ( O ) r " r , r / 1 ( N i ) r " r b" rc l o w 0 . 0 4 ) , t h e s u r f a c c h y d r o x y l a t i o n i s m o r c i n r p o r t a n t .T h i s u n c x p c c t c d
r e s u l t m a y b e r e l a t e d t o t h e e x p e r i n r e n t acl o n d i tions: the lowest exposures wcre pcrformcd at
P o r : 7 x l O - i m b a r ( i n s t e a do f I x 1 0 - 6 )s o t h a t
the partial pressureof residualwatcr was proportionally higher.
The data obtained for various oxygenexposurcs
are presentedin Fig. l; the XPS intensity ratio
1(O)t"t"l//(Ni),o,u,,obtained from the integration
o f t h e O l s a n d N i 2 p 3 7 c2o r e l e v e l s i,s p l o t t c d a s a
function of the O: exposurcexpressedin langmuirs
( L ) . I n F i g . I , a f i r s t p l a t e a u( c o r r c s p o n d i n g
to the
o x y g e n a d s o r p t i o n p h a s e ) i s n o t e v i d e n c e d ,a s
c o m p a r e dw i t h w h a t i s r c p o r t c d i n t h c l i t c r a t u l c
[a0]. This is not rclatcd rvith thc tcrnpqraturcof
a d s o r p t i o n :l i t e r a t u r ed a t a s l i o r v t h a t t h c h i g h e r
the temperatureof adsorption, the longer is the
first plateau in the oxygen uptake curve. This
might be attributed to a lack of data at the lowest
oxygen exposures,but it is more likely correlated
w i t h o u r e x p e r i m c n t apl r o t o c o l :o x y g e ne x p o s u r e s
a n d s a m p l e h e a t i n g i n t h e p r e p a r a t i o nc h a m b e r ,
1an
160
E
t.+U
r
ltn
E
roo
o
5qn
z
Y^^
:
.+u
20
n
0
200 400 600 800 1c00 1200 1400
(L)
02exposure
F i g . l . O l s i n t e n s i t y ( n o r m a l i s e dw i t h r e s p e c tt o t h e N i 2 p r 7 z
core level intensity) as a function of oxygen exposure (expressed
in Langmuir) at 650 K.
a n d X P S m e a s u r e m e n tisn t h e a n a l y s i sc h a m b e r .
Indeed, the different surface compositions w€re
"frozen" in the preparation chamber, by stopping
the sample heating, but the temperaturedecrease
was not instantaneous.
Simultaneously,oxygenwas
pumped away. However the pressuredecreasewas
gradual, so the true 02 exposul'eis always slightly
h i g h e rt h a n i n d i c a t e d C
. o n s e q u e n t l yn, o t a l l o f t h e
exposurewas performed at constant temperature
a n d p r e s s u r e ,s o o L l r r c s u l t s c a n n o t b e d i r e c t l y
c o m p a r e d w i t h t h e l i t e r a t u r ed a t a a t I < 5 0 0 K .
Moreover, a changein the surfacelayer structure
during the sample cooling cannot be excluded:it
has been shown that thin oxide films, formed on
N i ( l I l ) a t 5 0 0 K , d e c o r n p o s er a p i d l y u n d e r a n nealing at 550 K [47].
To investigatethe role of oxygen in the interaction of N H3 with Ni( I I I ), it appearednecessary
to
characterizethe NiO growth mode in our experimental protocol, using our XPS data. For this
p u r p o s e ,w e h a v cc o n s i d e r c dt w o m a i n h y p o t h e s e s :
c i t h c r t h c o x i d c f o r m a t i o n p r o c c c c l sv i a a " l a y c r b y - l a y e r " g r o w t h m o d c o r i t c o r r e s p o n d st o a
" t w o - p h a s c d o m a i n " ( n u c l c a t i o na n d g r o w t h o f
N i O i s l a n da m o n gt h e O - a d s o r b e dp h a s e ) a, s i n t h e
k i n e t i cc o n d i t i o n sb e t w c e n3 0 0 a n d 5 0 0 K . I n t h i s
approach, the Ni2p312core levels have been decomposedsystematicallyinto the metallicand Ni2*
(in NiO) features,and the oxidic contribution at
529.8eV has bcenextractedfrom the O 1score level
E, Luksono at ul. I Surfucc Sciantc 530 (2003) 37-54
Niosatelhle
p€ak
NOmarn
8544 eV,urrarrpeak
Ni
Nodouoer
b b r e v n'rr=u.u e s o l ' v I
"o"l'lt]ft
i56ili
,ta
,8s3oev
o
I3U
Z
1An
'*
- "layer layer"model
by
. experimental
results
=
CJ
=x
OJU
En
o
:
468101214
16 18
(x100)
l(O)oxide
/ l(Ni)total
(a)
F .' _ _ _a_n_\ / e r a g e
O-edcnrhod
0.8
€o
nh2ca
0.6
04
0.2
o
>
o
p
o
o
tcu
a
=
,n
d><
- "two-phase
domain"
model
oYnarimani2l rpar'lla
o
p
xEn
OJU
535
(b)
533
531
529
(eV)
EnergY
Binding
527
525
Fig. 2. (a) Ni 2p3pand (b) O ls corelevelpeaksdecomposition
into the metallicand oxide fNiO) features,after 450 L of exposureof 02 with Ni(l I l) at 650K.
16.2 A for O in the (10O)-orientedoxide
( r . f i o ( ' 0 0 ) )I.t w a s a s s u m e dt h a t t h e a t t e n u a t i o n
of the nickel signal through an O-adsorbedmonoIayer can be negiected, and thus exp(-d" f
2fl,sin B) = L For both models, the following ratios have been used:
I ( o ) I i a .: n , , 7
4Ni;;,0"
/ ( N i ) n i ,:i "o R r s
/(Ni)r",,r
and
1(o)lXlll"to'ln"*
: o o4s
/(o)Iio.
Details on the calculations are rcportcd in the
Appendix. In Fig. 3a and b, calculated and exhave been
perimentalvalues of' /(O)"",0"/1(Ni)o^,,1.
superimposed.One can easily observe that the
"layer-by-layer" model does not fit the data,
whereasthe "two-phase domain" model provides
a satisfactoryfit. Other valuesof ,i.fl,derivedfrom
:
n
(b)
024681012141618
(x100)
I(O)oxide
/ t(Ni)total
ratio,obtained
fromtheOls
Fig. 3. (a)ONi2*XPSintensity
core level and the Ni 2prrz core level for Ni2+ in NiO, as a
function of the total O/Ni XPS intensityratio, obtaincd from
t h e O l s a n d N i 2 p 1 7 2c o r e l e v e l s .P o i n t s : e x p e r i m e n t a lr e s u l t s ;
f u l l l i n e : l a y e r - b y - l a y enrr o d e l .( b t O , N i ' z - X P S i n t e n s i t yr a t i o ,
c o r e l e v e lf o r
o b t a i n e d f r o m t h c O l s c o r e l e v e la n d t h c N i 2 p 3 7 2
N i 2 + i n N i O , a s a f u n c t i o n o f t h e t o t a l O A I i X P S i n t e n s i t yr a t i o ,
o b t a i n e d f r o m t h e O l s a n d N i 2 p : , : c o r e l e v e l s .P o i n t s : e x p e r i m e n t a l r e s u l t s ;f u l l l i n e : t w o - p h a s ed o m a i n m o d e l .
more recent equations [49,50], have been tested:
they induce changesin the calculatedthicknessof
the nickel oxide layer but the shape of the curves
(Fig. 3a and b) remains unchanged. The main
reason for the small deviation of some experimental points most probably comes from the too
simplistic hypothesisof the isomorphic growth of
the oxide islandsin the very early stagesof the NiO
growth. We conclude that the two-phasedomain
model describedwell the growth of the NiO oxide
layer, in our experimentalconditions, and this result will be used for the following NH3 adsorption
studies.
E. Laksono et al. I Surfucc Sticnce 530 (2003) 37-54
3.2. NHr adsorption
In the presentwork, adsorption of ammonia on
cleanand oxygenpre-treatedNi(l 1 1) surfaceswas
performed at room temperature, in the preparation chamber of the spectronteter.A typical controlled exposure of immonia lasts 3 min at the
desired pressure. At the end of the treatment,
ammonia is pumped down to a pressureof l0-8
mbar, and the sampleis transferredto the analysis
chamber for XPS measurements.The sample can
then be re-exposedto ammonia under the same
conditions.
In the absenceof oxygen on the Ni(l l l) surface,no adsorption of NH3 was observedby XPS,
in agreementwith previous data [39]. After interaction of ammonia with the oxygen pre-treated
Ni(l I l) samples,two N-adspecieshave been detectedin the N 1s core level region: one located at
3 9 9 . 8+ 0 . 2 e Y a n d t h e o t h e r o n e a t 3 9 7 . 8* 0 . 2 e V .
A l l N l s c o r el e v c ls p c c t r ah a v c b e c ns y s t c m a t i c a l l y
decomposedinto a combination of thesetwo features.For a low NH3 exposure(13.5 L), if the XPS
analysis is performed just after the exposure to
ammonia, the N ls core level presents a marn
featureat high binding energy(HBE: 399.8eV). If
the same analysis is repeated after one night in
UHV, the ratio betWeenthe surface adspecicsis
significantlychangedin favour of the low binding
energyadspecies(LBE: 397.8eV). The situation is
different for cumulated NH3 exposuresfrom 40.5
L up to about 180 L: the N 1s core level presentsa
main feature at LBE and the XPS spectraremain
unchangedwith time in the analysischamber.
To focus on the initial stages ol' adsorption
for low ammonia exposures, the O-pre-treated
N i ( l I l ) s a m p l e sw e r e e x p o s e dt o N H r f o r 1 3 , 5L
in the preparation chamber, then the N ls signal
was measured as a function of time in UHV,
during about 500 min. The same procedure was
r e p e a t e df o u r t i m e s .F i g . 4 s h o w sa t y p i c a ls e r i c so f
N ls spectra obtained as a function of time in
U H V a f t e r e x p o s u r et o N H 3 ( X P S r a t i o 1 ( O ) , " , " , /
/(Ni)roor :0.022). Each N ls core lcvel peak has
been decomposedinto a combination of the LBE
and HBE components. Fig. 5 shows the results
obtained as a function of time, flor the two adspeciesand for the total N ls signal.After the first
A
E
G
c
c
U)
X
405
403
401 399 397
(eV)
Binding
Energy
395
F i g . 4 . C h a n g e si n t h e N l s s p e c t r u m a s a f u n c t i o n o f t i m e i n
U H V , a f t e r t h e f i r s t N H 3 e x p o s u r eo l ' 1 3 . 5 L a t r o o m t e m p e r a t u r e . T h e N l s p e a k d e c o m p o s i t i o ni n t o t h e H B E a n d L B E
c o m p o n e n t si s s h o w n . T h e N i ( l I l ) s u b s t r a t ew a s p r e - t r e a t e d
b y 0 2 a t 6 5 0 K , 3 0 s e c o n d sa t I x I 0 - ? m b a r . F r o m t h e f i r s t
(lower part) to the fourth spectrum, as well as from the fifth to
t h e l a s t s p e c t r u m( u p p e r p a r t ) , t h e t i m e b e t w e e nt w o s p e c t r ai s
4 5 m i n . B e t w e e nt h e f o u r t h a n d f i l t h s p e c t r a ,t h i s t i m e i s 9 0 m i n .
T h e t o t a l t i m e s c a l ei s - 9 h .
exposureto ammonia, the low BE signal increases
with time whereas the high BE signal decreases.
Moreover, the total NH3 uptake dccreaseswith
time, suggestinga partial desorption of the high
B E a d s p e c i e sA.f t e r t h c s e c o n da m m o n i ae x p o s u r e ,
tl.tc observations ale simrlar, with a less pronounced increasein the iow BE signal. After the
third dose, the surface distribution between the
H B E a n d L B E N l s f c a t u r e sr e m a i n sa l m o s t u n changed with respect to the last N ls spectrum
recordedafter the seconddose. However, the total
N ls area has slightly increasedcompared to the
seconddose. No lurther chanse is observedafter
E. Luksono et al. I Surface Science530 (2003) 37-54
48
u)
tn
.E
=
C
=
-i
E
L
>\
.;;
-
'i;.
r
',;
.c
a
><
n
X
528
q?)
532
(eV)
energy
Binding
(eV)
energy
Binding
0HI
(d)
U)
a
-ct
=
A
zt
"^-*
tJ)
c
VI
a
X
I-
*n-'
V,
^ N
oX
540
528
532
536
Binding
energy(eV)
//t
./ / [
///
-r ; /.
.r / |
,/ / /
-/.1
/
, / / /
J ) l
540
s2B
532
536
energy(eV)
Binding
e x p o s u r e( 1 3 . 5 L ) ' ( b ) t h e
F i g . 9 . C h a n g e si n t h e O l s s p e c t r u m a s a f u n c t i o n o f t i m e i n U H V o b t a i n e d a f t e r ( a ) t h e f i r s t a m m o n i a
e
x
p
o
s u r e( 1 3 . 5 L ) . B e t w e e nt h e
a
m
m
o
n
i
a
(
d
)
f
o
u
r
t
h
(
1
3
.
5
t
h
e
L),
s . " o n d a 1 n m o n i ae x p o s u r e( 1 3 . 5 L ) , ( c ) t h e t h i r d a m m o n i a e x p o s u r e
- 8 h f o r t h e s e c o n de x p o s u r e( b ) ,
(
a
)
,
6
e
x
p
o
s
u
r
e
t
h
e
f
i
r
s
t
h
f
o
r
i
s
:
(
t
o
p
)
t
i
m
e
s
c
a
l
e
t
h
e
s
p
e
c
t
r
u
m
,
l
a
s
t
a
n
d
t
h
e
(
b
o
t
t
o
m
)
spectrum
first
- 7 h f o r t h e t h i r d e x p o s u r e( c ) a n d - 8 h f o r t h e f o u r t h e x p o s u r e( d ) .
intensity ratios of these sut'facesafter the mentioned treatments.
After annealing under UHV, the Ni 2p372core
levelpeak exhibitsa main featurecorrespondingto
metallic nickel, larger than before annealing,and
the O ls core level peak area is lessintense,with a
shoulder at 531.5 eV, less pronounced compared
to the spectrum beiore annealing.From theseob-
E. Laksono er ul. I Surface Science530 (2003) 37-54
Table I
XPS intensityratiosobtainedfrom t h e O I s a n d N i 2 p 3 7 2c o r e
levelsafter 02 interactionat 650 K, before and after UHV
annealing
at 650K
Thin film I
Thin film 2
/(o),,,,,r//(Ni),.,,,r/(o)r,,,,r//(Ni),,,,.,r
B e f o r ea n n e a l i n g
Aftcr annealing
0.I 5
0.12
0 .r 3
0.05
groups as a function of time, according to the
following reaction:
N H 3 ( a d s )+ . r O ( a d s ) -
NHr-*(ads)+ xOH(ads)
x : \,2
( s c h e m eI )
The value of x will be discussedbelow.
3.3. Discussion
servationson the core levels,different hypotheses
c a n b e c o n s i d e r c di n v o l v i n g a d c c r e a s ei n t l i c
quantity of nickel oxide zrndior a decreasein the
o x i d e s u r f a c e c o v e r a g e :( i ) c i t h e r a d c c r e a s ei n
the oxide islands tl.rickness,the oxide covcragc
remaining constant; or (ii) a decreasein the oxide
coverage,the oxide islands thickness being constant; or (iii) a decreasein the covcrageof oxide
with an increaseof the oxide island thickness;or
(iv) both a decreasein the oxide coverageand the
oxide thickness.
As regards the surface reactivity towards ammonia, it was observedin both casesthat the adsorption was greatly enhanced compared to the
surfacebefore annealing.For Thin Film l, corres p o n d i n gt o 1 ( O ) r o r o , / / ( N i ) , o:, , r0 . i 2 ( s e eT a b l e l ) ,
the XPS intensity ratio after ammonia adsorption
b e c o m e s / ( N ) / / ( N i ) r " r u: r 9 X l 0 - 4 * I x 1 0 - aa n d
for thin film 2, the ratio is 1(N)/1(Ni),o,u,: 14 x
l 0 - 4 + I x l 0 - 4 . B a s e d o n o u r p r e v i o u so b s e r v a tion of the absenceof reactivity of nickel oxide in
our conditions (Fig. 7), this new seriesof experiments confirms the enhancedreactivity of the Oadsorbedphaseon the surfaceof the sample.In the
particular caseof Thin Film 1, in spite of a small
(from
decreaseof the XPS ratio ^/(O),o,u,/./(Ni),o,",
0
.
1
2
)
,
a
m
m
o
n
i
a
a
d
s
o
r
p
t
i
o
n
0.15 to
the
enhancement suggestsa decreasein the covelageof oxide
with an increaseof the oxide island thickness(hypothesis (iii)): not only the NiO(l00) phase decomposed into the O-adsorbed phase but also a
local thickening of the oxide islands occurred, as
alreadyobservedwith Ni substrates(1 0 0)-oriented
l,s2l.
At this point, to summarisethe results,we have
observedthat NHj reactswith the Ni(l I l) surface
provided some O-adsorbedphase is present;once
adsorbed, molecular NH3 reacts u'ith oxygen to
form dissociatedammonia speciesand hydroxyl
W e { i r s t c o m m e n to n t h e b i n d i n gen e r g i e so f t h e
N - a d s o r b e ds p c c i e s .I n t h i s ' " v o r k , t w o d i f f e r e n t
v a l u e sh a v eb e e nm e a s u r c dt:h e L B E a t 3 9 7 . 8+ 0 . 2
e V , a n d t h e H B E a t 3 9 9 . 8* 0 . 2 e V . S o m es e l e c t e d
v a l u e so f N 1 s B E , f r o m t h e l i t e r a t u r e a
, rereported
i n T a b l e2 .
To focus more specificallyon the comparison
with previous works on ammonia adsorption at
room temperature on oxygen pre-treated nickel
single crystals, Grunze et al. [29] have already
faced the difficulty of unambiguously identifying
the nature of a NH3 adsorption complex on an
oxygen pre-covered Ni(l 0 0) surface. They observeda broad peak centred at 399.6 eV, but did
not attempt to decomposeit and assignedit to
NH3 or NH:. HO. In the work of Kulkarni et al.
[ 3 3 ] o n o x y g e np r e - t r e a t e dN i ( l 0 0 ) a n d N i ( l l 0 ) ,
the broad N ls spectra obtained after ammonia
exposure at 300 K, centred at 398.0 eV, were
Table 2
C o m p a r i s o n o f N l s B E f o r a d s o r b e dn i t r o g e n s p e c i e so n d r f f e r e n t m e t a l sa n d o x i d e s
Adsorbed nitrogcn specics
N ls BE (eV)
Referencc
BN on Ni(100)
NI-lr on Ni(l I 0)
NH3 on Ni(l 00)
398.5
400.9
400.5
400.2and
399.8(second layer)
400.1
400.7
398.7-399.4
3e8.6
398.0-398.6
3e8.5
397.7
398.4
396.G397.0
391.0
t53l
[ ]l
t33l
t29l
NHr on Ae(l I l)
NHr on Cr:Or/Cr(l l0)
N H 2 o n d i f f e r e n tm e t a l s
NH2 on Cr:Or /Cr(l l0)
N H o n d i f f e r e n tm e t a l s
NH, (.r: 1,2)on Ni(l 00)
NH on Ni(100)
NH on Ni(l I 0)
N o n d i f f e r e n tm e t a l s
N onNi(I00)
[54]
t38l
[54]
[38]
[54]
[33]
I29l
[ 13 ]
[54]
t3ll
E. Luksortoet ul. I SurfaceSciance530 (2003)37-54
minority of the O-adsorbedatoms are in states3a
(or 3b) and 3c. The majority of oxygen is in state
3d. Moreover, the extrapolation of the data of Fig.
5 to t = 0 indicatesthat about 200hof the molecular adsorbed ammonia has desorbed in UHV
conditions.In state 2, the atomic O/NH3 ratio can
be roughly estimated to 4ll, which means thar,
even before the ammonia desorption and conversion stage, all the equivalent fcc hollow sites are
not occupiedby molecular ammonia. The possible
electronicor stericeffectshindering the adsorption
of ammonia in some fcc sitesare not elucidated.
It is to be noted that the first hypotheses(states
I and 2) are the samc as those put forward by
Netzer and Madey [27], with adsorption in hollow
sites.With the samesitesfor the O-adsorbedphase
(state 1), another possibilitywould be to assignthe
molecular ammonia of state 2 to top sites,except
the"onesthat are nearestto the adsorbedoxygen.
This configurationleadsto the samealternativefor
the NH2 sites(states3a and 3b in Fig. l0). Theoretical calculations might help in distinguishing
which hypothesisis more likely.
4. Conclusions
The general features that can be derived from
the above resultsare as follows.
As regards the interaction of oxygen with
Ni(l I l) at 650 K, it was shown that, under our
experimental conditions, the surface oxidation
proceedsvia a "two-phase domain": O-adsorbed
phaseand NiO islands coexist in a large range of
oxygen exposure.
As regardsthe surface reactivity towards NH3,
a t r o o m t c m p e r a t u f ca n d a t I x l 0 - 7 m b a r . t h c
conclusionsare the following:
(i) There is no adsorption on the clean Ni(l I l)
surface.
(ii) The surface reactivity is strongly correlated
with the presenceof O-adsorbedphase coverage:
the higher the 0o6, coverage, the higher the
a m o u n t o f a d s o r b e da m m o n i a .
(iii) A continuous nickel oxide thin layer is not
reactivein our conditions.
(iv) Two N-adspecieshave been detectedfrom
t h e N 1 sc o r e l e v e lp e a k ,a t 3 9 9 . 8+ 0 . 2 e V ( h i g h B E
feature) and at 397.8+ 0.2 eY (low BE feature),
and assignedto molecular NH3 and dissociated
NH,(,=r.z)species,respectively.
( v ) F o r l o w a m m o n i ae x p o s u r e st,h e d i s s o c i a t e d
ammonia adspeciesis formed from a fraction of
t h e m o l e c u l a r a d s p e c i e sa, n d c o n c o m i t a n t l y ,t h e
h y d r o x y l c o m p o n e n ti n t h e O l s c o r e l e v e l p e a k
increases,while the main feature at 529.9eV dec r c a s e s .T h i s s i m u l t a n e o u st r a n s f o r m a t i o n h a s
evidencedthe hydrogen abstraction of ammonia
by adsorbed oxygen to product OH and NH,. A
quantitative treatment of the XPS data gives the
following stoichiometry:
N H 3 ( a d s )+ O ( a d s )-
N H z ( a d s )+ O H ( a d s ) .
(vi) From our experimental results, it appears
that the kineticsof desorptionof ammonia is faster
than the kinetics of dissociation.
(vii) The total amount of adsorbedammonia at
saturation is about five times smaller than the
amount of O-adsorbedspecies.At equilibrium, the
ratio betwecn molecular NH3 and NH2 adspecies
is l;4.
(viii) The behaviour of thc two nitrogen adspeciessuggeststhat two differentadsorption sitesare
required.
Appendix
Calculatiottof the XPS intensityraliosfor the " tvophase domain" and the "layer-by-layer" models
( l ) The 1(Ni );id" // (N i )T.,,r and / (o )il"::llvd';h","
/
1(O)I,0" XPS ratios are respectivelyexpressedby:
- 1(Ni)::id:r'[r
1(Ni);id. _
INnil
- '-n(-
N.-r-)]
,(NmF-c,xP,Jl
and
onolaycr
;161monolalcr
'/ 1\ "n,\i m
a d s o r b c dp h a s c_ '
\"/adsorbcd phasc
I(O)Ld"
r( o):l;:"'
.[,
-'-e(,*-h)
E. Luksono ar ul. I Surfuca Scianca530 (2003) 37,54
where:r is the thickness
of one NiO(I00) layer
( : 2 . 0 8 6 A ) a n d/ ' i s t h et h i c k n e sosf o n eN i ( l I 1 )
Iayer(:2.036 A).
A"sumingthat the photoelectronemissionby
one atom (crosssection)is independentof its
chemicalenvironment,
the
/(Ni):ffifv" .._r 1(o)i:l:i:'d";h.,.
srru
-
/(Ni)a:l?v"
r(o)::io'j"'
ratios are equivalentto the ratios of the number of
atoms in one layer for each compound. For Ni
(l I l)-oriented,NiO (1 0 0)-orientedand a p(2 x 2)
O-adsorbedohase.we obtain
,r(Ni):lidr:v.'
_ ^ ,.1.7 a n O,
.r(Ni)il:jiv.,
'/^\monolaver
1(u.)"drorb.i oh.,r.
-
-/
= U.+U)
/ o- ) o/ on\ ted er r ) e r
\
which leads to
/ (o)I",l?l'.'o'i,,,,,
: 0.049
and
1(o)I'0.
ffi#:08r5
(2) To obtain the ,r(O)fr,."/1(Nifiid"ratio, we
haveusedthe generalquantitativeformula:
I(M)ff oyDlnlflr$E)
/ / p\""
op$ ).!r@E)
where:M, P are two differentelements,N is the
matrixformedby M andP, D is the atomicdensity
of M or P in the matrix N, and o is the Scofield
crosssection[51].
For the VG ESCALAB Mk ll spectrometer,
);;tf WE)lA:;I(KE) is constant,which leadsto:
I(M);
ouDlt
.il
-z,'^r
t ^N -t,.-t
7m- or4
w i t h D U i o : D N i o ( : 0 . 0 9 1 6 m o l / c n . r 3 )o, 6 1 ,
2 . 9 3 a n d o N i 2 p r , r :1 4 . 6 1 ,t h e c a l c u l a t i o nl e a d st o :
1 ( O ) n i d . / 1 ( N i ) I i o .: 0 ' 2 0 t 0 ' 0 3 , a s s u m i n ga n u n certainty of 5o/uon the values of a mentioned by
Scofield [51]. Within this range, the best fit between the experimentalresultsand the theoretical
m o d e l i s o b t a i n e df o r / ( O ) f r , d . / 1 ( N i ) n i d :. 0 . 2 2 .
Rcfcrenccs
Surf.Sci.447(2000)
81.
Ul J.W.Klaus,S.M.George,
[ 2 ] J . S t o b e r ,B . E i s c n h u t ,G . R a n g c l o v ,T h . F a u s t c r ,S u r f . S c i .
3 2 r0 9 9 4l )r r .
)J
[ 3 ] A . A . S a r a n i n ,O . L . T a r a s o v a ,V . G . K o t ) j a r , E . A . K h r a m t s o v a , V . G . L i f s h i t s ,S u r f . S c i . 3 3 1 - 3 3 3( 1 9 9 5 )4 5 8 .
[ 4 ] C . B a t e r , M . S a n d e r s ,J , H . C r a i g J r . , S u r f . S c i . a 5 l ( 2 0 0 0 )
226.
z ,h i n S o l i dF i l m s3 4 7( 1 9 9 9 1) 9 5 .
[ 5 ] V . M . B e r m u d eT
Chem.Phys.Lett. 317(2000)290
[6] V.M. Bermudez,
[7] H.L. Siew,M.H. Qiao,C.H. Chew,K.F. Mok, L. Chan,
C . Q .X u , A p p l .S u r f .S c i .l 7 l ( 2 0 0 19) 5 .
[8] M. Kaltchev,W.T. Tysoe,Suri.Sci.430(1999)29.
, . E . B r i d g ei,n : D . A . K i n g ,W . P .W o o d [ 9 ] R . M . L a m b e r tM
ruff (Eds.),The ChemicalPhysicsof Solid Surfaces
and
H e t e r o g e n e oCu as t a l y s i sv,o l . 3 B , E l s e v i e rA, m s t e r d a m ,
1 9 8 4p, p . 5 9 - 1 0 5 .
, .P.Merill,
[ 0 ] C . W . S e a b u r yT, , N . R h o d i n ,R . J . P u r t e l l R
Sur[S
. c i .9 3 ( 1 9 8 0I)l t
I l ] C . R .B r u n d l eJ,. V a c .S c i .T e c h n o ll.3 ( 1 9 7 63) 0 1 .
, . S .J c n s e nT,. N . R h o d i n .R . P . M c r r i l l ,S u r f .
[ 2 ] K . J a c o b iE
S c i .1 0 8( 1 9 8 13) 9 7 .
, . G o l z eR
, . K . D r i s c o l lP, . A .D o w b e nJ,. V a c .
[ 3 ] M . G r u n z eM
S c i .T e c h n o l 8
. f l 9 8 l )6 l L
, . E .M i t c h e l lS, u r f .S c i .9 9 ( 1 9 8 05) 2 3 .
! 4 1 B . A .S c x t o nG
l l 5 l G . B .F i s h e rC, h c m .P h y s L. c t t . 7 9( 1 9 8 14) 5 2 .
[6] D.M. Thornburg,R.J. Madix, Surf. Sci. 220 (1989)
268.
[ 7 ] D . C h r y s o s t o m oJu., F l o w e r s , F . Z a e r a , S u r ' f . S c i . 4 3 9
(t999) 34.
J, Stohr,T K e n d e l e w i c z ,S u r f . S c i , 1 3 4 ( 1 9 8 3 )
[8] R. Jaeger,
547.
R.M. van Hardeveld,R.A. van
[9] J.W. Niemantsverdriet,
Santen,Surf.Sci.369(1996)23;
R.M. van Hardeveld,R.A. van Santen,J.W. Niemantsverdriet,J. Vac.Sci.Technol.A l5 (1997)1558.
[20]W. Widdra,T. Moritz,K.L. Kostov,P. Konig,M. Staufer,
U . B i r k e n h e u eSru, r i ,S c i .L c t t .4 3 0( 1 9 9 9L) 5 5 8 .
[ 2 1 ]C . B a t e rJ, . H . C a m p b e lJl ,. F I .C r a i gJ r . ,T h i n S o l i dF i l m s
340fl999)7.
H. Kubo, M. Ito, Surf.9ci.427a28(1999)
[22]Y. Shingaya,
t73.
J.M. White,Surf.Sci.406(1998)194.
[23] G.J.Szulczewski,
[ 2 4 ]Y . Z h e n gE, . M o l e r ,E . H u d s o nZ, . H u s s a i nD, . A . S h i r l e y ,
P h y sR
. e v .B 4 8 ( 1 9 9 34) 7 6 0 ,
r ,. F r i t z s c h e
M, . C . A s e n s i oP, . G a r d n e r ,
[ 2 5 ]K . - M . S c h i n d l cV
D . E . R i c k e n ,A . W . R o b i n s o n A
, . M . B r a d s h a wD
, .P.
WoodrufT,
J.C. Conesa,A.R. Gonzalez-Elipe,
Phys.Rev.
B 46 0992\ 4836.
K. Wagemann,
J. Kiippers,G. Ertl, Surf.
[26]I.C. Bassignana,
(1986)22.
Sci.175
l27l F.P.Netzer,T.E. Madey,Surf.Sci.ll9 (1982)422.
[28]T.E. Madey, C. Benndorf,Surf. Sci. I52-153 (1985)
587.
[29] M. Grunze,P.A. Dowben,C.R. Brundle,Surf. Sci. 128
( r 9 8 3 3) i l .
Chem.Phys.Lett.'74(1980)472.
[30]C.T. Au, M.W. Roberts,
[ 3 1 ]M . W . R o b e r t s ,A p p l . S u r f . S c i . 5 2 ( 1 9 9 1 )1 3 3 , a n d
references
therein.
M.W. Robcrts,D.
[32] B. Afsin, P.R. Davies,A. Pashusky,
V i n c e n tS, u r f .S c i .2 8 4( 1 9 9 3 I)0 9 .
54
E. Luksonoet al. I SurfaceScience530 (2003)37-54
[33]G.U. Kulkarni, C.N.R. Rao, M.W. Roberts,J. Phys.
Chem.99(1995)3310.
[34]H.E. Dastoor,P, Gardner,D.A. King,Surf.Sci.289(1993)
279.
E. Legsgaard,F. Besenbacher,
[35]L. Ruan, I. Stensgaard,
Surf.Sci.314(1994)L873.
, . W . G o o d m a nJ,. P h y sC
. hem.
[ 3 6 ]M . - C .W u , C . M .T r u o n gD
97 (t993) 4182.
[37]R.S. Mackay, K.H. Junker,J.M. White, J. Vac. Sci.
Technol.A 12(1994)2293.
P. Marcus,Appl. Surf.Sci.I53 (1999)
[38]H. Ma, Y. Berthier,
40.
E. Laksono,C. Argile,P. Marcus,Surf.
[39]A. Galtayries,
, Interf.Anal.30 (2000)140.
[40]C.R. Brundle,J.Q. Broughton.in: D.A. King, W.P.
Woodrufl(Eds.),Thc ChemicalPhysics
of SolidSuriaccs
and Heterogeneous
Catalysis,
vol. 3A, Elsevier,
Amsterd a m ,1 9 9 0p, p . 3 5 1 - 3 7 0 .
[41]P.R.Norton,R.L. Tapping,FaradayDisc.Chem.Soc.60
(1975)71.
J.K. Norskov,Prog.Surf.Sci.a4 (1993)5.
[42]F. Besenbacher,
, . H . K i m , S u r f .S c i .4 5 9 ( 2 0 0 0 )
[ 4 3 ]C . M . K i m , H . S . J e o n g E
L457.
R. McGrath,F. Borgatti,M.
[44]P. Finetti,M.J. Scantlebury,
Sambi,L. Zaratin,G. Granozzi,Surf.Sci.46l (2000)240.
[45] F. Rohr, K. Wirth, J. Libuda,D. Cappus,M. Batimer,
H . - J .F r e u n dS, u r f .S c i .L c t t .3 1 5( 1 9 9 5L) 9 1 7 .
V. Maurice,C. Ilinnen,P. Marcus,Surf.Sci.
[46]N. Kitakatsu,
407 (1998)36.
[ 4 7 ]N . K i t a k a t s uV, . M a u r i c eP, . M a r c u sS, u r l lS c i . 4 l l ( 1 9 9 8 )
215.
Spectroscopy,
[48] M.P. Seah,Augerand X-ray Photoelectron
Practical
Analysis,
vol. I, Wiley,Chichester,
Surface
1990,
pp. 201-255.
M.P.Seah,Surf.Interf.Anal.25(1997)430.
[49]P.J.Cumpson,
[50]S. Tanuma,C.J.Powell,D.R. Penn,Surf.Interf.Anal. 25
(1997)25.
J. ElectronSpectrosc.
8 (1976)129.
[51]J.H. Scofield,
R.
Kubiak,
IL
Morgncr,
O,
Rakhovskaya,
SurL Sci.321
[52]
(1994)229.
, . J .G e l l m a nS, u r f .S c i . 3 8 2
[ 5 3 ]R . M . D e s r o s i cDr ,. W . G r c v e A
(1997)35.
[ 5 4 ]A . L . S c h w a , n eEr ., D . P y l a n t ,J . M . W h i t e ,J . V a c . S c i .
Technol.A l4 (1996)1453.
, . J . C . Sv.a n d e
[ 5 5 ]W . B i e m o l tA, . P . J J. a n s e nM, . N e u r o c kG
K e r k h o fR
, . A . v a nS a n t e nS, u r f .S c i .2 8 7 - 2 8 (81 9 9 3 1) 8 3 .