Interaction of atomic nitrogen with Rh(110)

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Surface Science 276 (1992) 144-155 North-Holland

Interaction

._._;

i.

surface science I :..:.::.:: ... ., ‘. ................

.A. .,... ~.:~.‘:,.‘::i::

of atomic nitrogen with Rh( 110)

S. Lizzit ‘, G. Comelli

2, Ph. Hofmann

2,3, G. Paolucci

2, M. Kiskinova

2 and R. Rosei

‘v2

’ Dipartimento di Fisica, Universitci di Trieste, 34127 Trieste, Italy 2 Sincrotrone Trieste, Padriciano 99, 34012 Trieste, Italy 3 Fritz-Haber-Institut der Max-Planck-Gesellschaft, D-1000 Berlin 33, Germany

Received 26 March 1992; accepted for publication 15 June 1992

The behaviour of atomic nitrogen layers on a Rh(llOJ surface produced by dissociative adsorption of NH, at 430-450 K or by atomization of N, on a hot filament was studied by means of thermal programmed desorption (TPD), low energy electron diffraction (LEEDS and Auger electron spectroscopy (AES). Combined LEED, TPD and Auger measurements were used to investigate the growth, ordering and stability of the nitrogen layers. Two ordered phases 3 x 1 and 2 x 1 were observed with increasing nitrogen coverage. The transition from a disordered layer at very low coverages, to ordered phases at moderate and high coverages, resulted in apparent higher temperature shift of the N, TPD traces indicating stabilization of the adlayer. The desorption parameters corresponding to the disordered, 3 x 1 and 2 X 1 phases were evaluated from the desorption data using leading edge lineshape analysis of the TPD traces and desorption at constant temperature. It was found that at moderate and high coverages the desorption is best fitted to a first-order desorption kinetics. The desorption energy increased by * 40 kJ/mol as a result of ordering of the layer. Comparison of the TPD, Auger and LEED data for various nitrogen layers indicates that a fraction of nitrogen adatoms penetrates beneath the surface and desorbs after liberation of the surface sites in a wide temperature range (550-800 K). Several models including possible N-induced structural changes of the surface are proposed to explain the structure and stability of the nitrogen layers.

1. Introduction

Compared to oxygen the interaction of nitrogen with Group VIII fee transition metal surfaces has been studied far less intensively. The adsorption of molecular nitrogen on Ni, Rh, Pd, Ir and Pt is weak so that it can occur only below room temperature [l-3]. Warming up of the adsorbed layer does not lead to dissociation because of the high activation barrier for dissociation [4]. Nitrogen adatoms can be deposited on the transition metal surfaces under consideration in different manners, e.g. by atomization of N, using a high frequency discharge [5], a hot filament [6,7], an ion gun [g-10] or by dissociative adsorption of gases containing N and further removal of the undesired species by simple desorption or secondary reactions [ 1l-131. Most extensive studies have been performed on Ni and Pd single crystal surfaces where N 0039-6028/92/$05.00

adatoms are rather strongly bound on the surface [2,3]. In recent studies of N/Ni(llO) [lo,121 and N/Pd(llO) [9] systems it was reported that a (2 x 3) LEED pattern is formed at elevated temperatures. The same LEED structure induced by N adatoms on Cu(ll0) has been described in terms of a surface reconstruction with each third [liO] row missing [14]. For N/Cu(llO) the double periodicity in the [IlO] direction was associated with N adatoms located in long-bridge sites. To our knowledge there are only few reports on nitrogen atomic adsorption on polycrystalline Rh surfaces [6,15] and on Rh(ll1) [161, where the N, molecule was atomized by a hot W filament or by microwave discharge but no detailed evaluation of the structure and stability of the adsorbate layer was made. We present here TPD, LEED and Auger results obtained for the system atomic nitrogenRh(ll0) where nitrogen adlayers were produced

0 1992 - Elsevier Science Publishers B.V. All rights reserved

S. L&it et al. / Interaction

of atomic nitrogen with RhtllU)

14.5

by exposing the surface to NH, at elevated temperatures or by atomization of N, gas molecules by a hot W filament.

2. Expe~mental The experiments were performed in an UHV system, equipped with a VG Spectaleed rear view LEER system, an Auger electron spectrometer with a VG Escalab Mk II analyser, and a Leybold Inficon QX2000 mass spectrometer. The high transmission of the electron energy analyser, combined with multichannel detection, allowed us to collect Auger spectra in the counting mode in a time of 5 min with a sample current of 10 nA. The spectra were ratioed to spectra of the clean surface, the background was fitted with a polynomial, and the Auger peak was integrated to yield Auger intensity. The sample was prepared by cycles of ion bombardment (15-20 ,uA/cm’ at an ion energy of 2000 eV) and heating in oxygen (pressure 5 x lo-* mbar) at 1100 K followed by flashing to 1400 IS and reduction in H, (5 X low7 mbar, 800 IS> in order to remove residual oxygen. This cleaning procedure results in a sharp 1 X 1 LEED pattern and Auger spectrum where no impurities were observed within the detection limit. Due to the overlap of the C and Rh Auger peaks the lack of carbon contamination was proved by the absence of a high temperature CO desorption peak after oxygen adsorption at room temperature 1171. More details on the apparatus and the cleaning procedure have been given elsewhere [18]. The deposition of N atoms on the Rh(l10) surface was performed in two manners: by dissociative adsorption of high purity NH, or by atomization of N, molecules on a hot W filament mounted N 5 cm in front of the sample. A simple beam doser was used for NH, exposures which ensured an effective increase of the flux onto the sample by more than 10 times. The production of enough atomic nitrogen on the hot filament required pressures of - (l-2) X 10e5 mbars and during these exposures the chamber was pumped only by a liquid N, cooled sublimation pump. Because of these conditions, dosing nitrogen via a

300

400

500

600

700

800

Temperature (K) Fig. 1. TPD spectra of H, (mass 21, NH, (mass 17) and N, (mass 28) measured after an exposure of 26~ 10e6 mbar*s to NH, at 350 K. Heating rate 2.5 K/s.

W atomizer produced a CO-contaminated layer. Therefore we used labelled Ni5 in order to distinguish between the CO and N, desorption traces. Thermal desorption experiments were performed with a linear heating rate of 0.5 and 2.5 K/s, the ramp being controlled by an “Eurotherm” PID device. In order to rule out any contribution of residual CO to the N, TPD spectra we always monitored simultaneously masses 28 and 14 and compared the traces and the integrals of the corresponding spectra.

3. Results 3.1. Thermaldesorption data Fig. 1 shows NH,, H, and N, TPD spectra taken after exposure of Rh(ll0) to NH, at 350 K. These spectra indicate that both intact desorption and decomposition of NH, take place during heating, where N is the most stable species on the surface. On the basis of this result we chose 430-450 K as the most appropriate temperature range for production of a NH,- and H-free N adlayer. This was confirmed by the absence of any NH, and H, traces in the TPD spectra taken after exposure to NH, at 430-450 K.

S. Lizzit et al. / Interaction

146

Fig. 2 displays a set of N, thermal desorption spectra (m/e = 28 and 14) taken following increasing NH, exposures at 430 K. The shape of these desorption traces is complex and there is a general trend of a shift towards high temperatures with increasing N coverage. With the exception of the traces observed in the limit of very low coverages, the shape of the TPD curves indicates that the associative N, desorption cannot be described as a simple second-order recombination process. Thus it was necessary to determine independently the order of desorption. More detailed examination of the TPD results revealed the following features. At very low coverages the TPD curves contain a peak with a maximum at - 480 K which is overlapped by a growing higher temperature peak. The contribution of the 480 K state is negligible at moderate and high coverages. This might be due to a coverage induced shift to higher temperature or to replacement of the 480 K state by a new one. At moderate and high coverages the shape of the desorption curves is dominated by two overlapping features. As will be described below, the relative contribution of these states determines the width and the tem-

0 06

I

m

400

450

500

550

Temperature

600

650

700

(K)

Fig. 2. N, TPD spectra for increasing exposures to NH, 430 K: (a) mass 28; (b) mass 14. Heating rate 2.5 K/s.

at

of atomic nitrogen with Rh(ll0)

350

400

450

500

550

Temperature

600

650

700

750

(K)

Fig. 3. (a) N, TPD spectra from saturated N layers produced at 430 K for: (1) freshly cleaned sample; (2) after several NH, adsorption/desorption experiments. (b) N, TPD spectra for saturated layers produced at different temperatures: (1) NH, dosed at 320 K, (2) NH 3 dosed at 450 K.

perature maximum of the major TPD curve. The apparent shift to higher temperatures with increasing nitrogen coverage is caused by the growth of a state at - 535 K, indicated by the arrow in fig. 2a. A very broad, weak feature also can be distinguished at temperatures above 550 K, but its contribution to the total intensity of the spectra is very small. Examination of several series of TPD data obtained after deposition of a N layer at 430 K showed that the relative contribution of the two overlapping high temperature states in the 500550 K range depended strongly on the history of the sample, e.g. whether the sample was freshly cleaned and checked for C traces by an oxygen adsorption/ desorption run or it had already been involved in several NH, adsorption/ desorption cycles. As a result, the peak maximum for the saturation coverage obtained under the same adsorption conditions (NH, dose, adsorption temperature and heating rate) can range between 523 and 542 K as shown in fig. 3a. We tentatively relate the result in fig. 3a to more effective ex-

S. Lizzit et al. f i~teraci~on of atomic nitrogen with Rh~ll~)

haustion of traces of subsurface oxygen (below our detection limits) during interaction with NH, which acts as a reducer. Fig. 3b illustrates that the location of the N, desorption maximum also depends on the temperature at which the layer was prepared. An increase of the adsorption temperature or anneal of a layer deposited at lower temperature lead to replacement of the desorption state at - 470 K, characteristic for desorption after saturation with NH, at 310-320 K, with higher temperature states. The difference between the N, TPD spectra in figs. 1 and 3b is due to the higher ammonia coverage that can be achieved with decreasing adsorption temperature. Note that after adsorption at 450 K the peak maximum in fig. 3b is at a higher temperature compared to the 430 K curves in fig. 3a. In summary the results in fig. 3 indicate that the growth of a more stable N layer is favoured by higher temperatures and/or in the presence of traces of subsurface oxygen. Both effects have been studied in detaii recently where the effect of oxygen is proved in coadsorption experiments 1193. In order to determine the reaction order of associative nitrogen desorption for the desorption states which dominate in the desorption spectra at moderate and high coverages, we performed a series of isothermal desorption measurements, For this purpose the sample with adsorbed nitrogen was heated to a certain predetermined temperature above the maximum of the initial lowest temperature state (- 480 K). We used the same linear heating rate as in the TPD experiments shown in fig. 2. At this temperature the ramp was stopped and the change in the desorption rate with time was monitored under isothermal regime. Auger measurements were carried out after adsorption of nitrogen and after completion of the isothermal desorption. They showed that for the chosen temperatures no nitrogen remained on the surface after a sufficiently long reaction time. This was confirmed by comparing the area under the major desorption peak (460-580 K range) in the TPD spectrum when the heating rate is uninterrupted with the areas under the isothermal desorption traces. The integrals under these traces turned out to be almost the same (with deviations of less than 10%). The removal of

147

2 0

10

20 30 40 Reaction time (s)

523 K 50

60

Fig. 4. Variation of the logarithm of the nitrogen coverage, MA,), with the isothermal reaction time for different desorption temperatures starting from a saturated nitrogen layer. The insert shows the Arrhenius plot where k is the rate at the corresponding temperature determined from the slope.

nitrogen in an isothermal regime at temperatures lower than that corresponding to the m~mum of the major TPD curve indicates that this adsorption state of nitrogen is completely depopulated above a certain critical temperature. This result allowed us to use directly the fractional areas, A,, under the isothermal desorption traces as a measure of the nitrogen coverage, 13, remaining on the surface after a reaction time, t. Fig. 4 shows ln(A,) versus reaction time plots constructed on the basis of the isothermal data. The fact that the plots in fig. 4 are straight lines proves that the nitrogen desorption at moderate and high coverages can be described as a firstorder process. The Arrhenius plot constructed using the ln(A,Xt) data and presented in the insert of fig. 4 gives Ed = 82 kJ/mol. For evaluation of the desorption kinetic parameters we used full lineshape analysis applied to several series of TPD spectra, as that presented in fig. 2. This method consists of plotting the variation of the ln(AP/l AP dT) with l/T for each of the TPD traces, where AP and fAP dT are proportional to the desorption rate, d@/dt, and the adsorbate coverage, 8, respectively. This allows one to examine the variations in the desorption energy, E,, and pre-exponential factor, v,, with adsorbate surface concentration

S. Lizzit et al. / Interaction of atomic nitrogen with Rh(llOl

148

when the desorption order is known [ZOI. Since Ed and pd can be both coverage and temperature dependent and since there are also contributions of more than one state to the TPD traces we used only the Ieading edge region at the onset of desorption where the coverage drops by less than 10% and the points of the Arrhenius plot lie on a straight line. The results obtained from three TPD series, using the established above desorption order of one, are shown in fig. 5. If at very low nitrogen coverages the desorption order is different from one only the values of the pre-exponential factor will be affected. There is a certain spread of the values which can be related to the change in the weight of the overlapping peaks which, as pointed out above, depends strongly on the history of the sample. Despite this spread it is quite obvious that up to about one half of the saturation coverage the desorption energy and pre-exponential factor increase with increasing nitrogen coverage reaching almost a constant value at coverages above 0.5. The changes in the desorption parameters with nitrogen coverage can be related to the observed transitions to ordered structures, as described below (see fig. 7). Better quality TPD spectra where one can resolve the overlapping states were produced by interrupted isothermal desorption experiments. The TPD spectra obtained after saturation at 430 K and partial desorption at 483 K are shown in fig. 6. The corresponding E, and V~ values are summarized in table 1A. They can be more preTable 1 Normalized N coverages, e/O,,, desorption periods of isothermal desorption, t

tM

@,‘@,,WW

@,‘@,,,(AES)

(A,) T(a) = 430 K, Treaclion = 483 K 0 1 1

87 242 336 645

0.55 0.44 0.36 0.22

0.64 0.4 0.34 0.15

(B) T(a) = 450 k; Treacria,,= 505 K 0 1 1

60 145 300

0.56 0.31 0.27

parameters,

0.87 0.38 0.06

0.0

0.20

0.40

0.60

0.80

1.0

1.2

Relative Coverage Fig. 5. Energy of desorption, E,, (a), and pre-exponential factor, vd (b), as a function of N coverage. PI,= 430 K. Different symbols show the values obtained from different desorption series. Heating rate: 2.5 K/s - filled symbols; 0.5 K/s - opened symbols.

cisely related to the actual structure of the layer. As will be discussed in section 3.2, this procedure produced better ordering in the layer as well. Table 1B presents the data obtained for a nitrogen layer produced upon adsorption at 450 K followed by partial desorption at 505 K. Note that

E, and v~, and structure of the N layers obtained after different time

E, (kJ/mol)

vd (s-l)

LEED

113 124 143 136 97

1.5 x 1 x 1 x 1.4 x 1.6 x

2X1 Streaky 2 X 1 MixedZXl&3xl 3x1 Streaks & weak h/3 spots

120 158 159 106

2 9 9 5

10’0 10”’ 10’3 10’2 10”

x 10’” x 10’” x 10’” x10*

2x1 Mixed2X1&3Xl 3x1 Very streaky 3 x 1

149

S. Lizzit et al. / Interaction of atomic nitrogen with Rh(ll0)

the Ed and v,, values for the mixed and 3 x 1 structures are higher when the adsorption and isothermal partial desorption were performed at higher temperatures, even taking into account that the accuracy of the evaluation procedure is +8 kJ/mol. The broad high temperature feature growing at T > 550 K is within the temperature range where N desorbs from a nitrogen layer produced using a hot W filament. It should be noted that the intensity of this state is sensitive to the procedure used for preparing the layer. As can be seen in fig. 6 its contribution increases when the layer is kept at higher temperature for a longer period. The desorption parameters of this state are briefly described in section 3.4. 3.2. Auger spectroscopy

M 28

400

450

500

5 0.6

-

.

CD G

0.4

-

0

0

.

.

10

50

60

Expos2uore(m361arseY.1 CP) Fig. 7. Nitrogen uptake curves at 430 K plotted using the integrals of the N, TPD (filled circles) and N(KLL) Auger (open circles) spectra.

data

The Auger measurements were exclusively used for determination of the nitrogen coverage and for isothermal desorption measurements. We made an attempt to calibrate the nitrogen coverage using as a basis the integral Auger spectra of N deposited together with oxygen via dissociative NO desorption. The calibration of nitrogen was performed by comparing the O(KLL) integral for oxygen adsorbed as a dissociation product of NO at 390 K with that of a saturated

350

0

0.6 -

550

Temperature

600 650 700 750 (K)

Fig. 6. N, TPD spectra obtained after partial desorption of nitrogen from a saturated layer produced at 430 K. Desorption temperature: 483 K. Reaction time: (a) 0 s; (b) 87 s; (c) 242 s; Cd) 336 s; (e) 645 s.

oxygen coverage deposited via dissociative oxygen adsorption at 300 K. In the latter case it has been found that the oxygen coverage is unity [21]. The difficulty arose from the fact that for the same N coverage the intensity of the N(KLL) signal was enhanced by the presence of 0 (by more than 50% at high 0 + N coverages). Thus, if we take into account the oxygen effect, the maximum N coverage achieved via dissociative NH, adsorption at 430-450 K should be of the order of or more than 0.5 ML (N atoms per surface atom). This value is in fair agreement with the N, TPD data for 0 + N and N layer where the integrals / AP dT of the corresponding TPD curves were compared. Here we would like to stress that because of the quite complicated influence of oxygen on the nitrogen Auger and TPD data the only definite conclusion is that the saturation N coverage achieved upon exposure to NH, is not less than 0.5 ML. In recent studies of NO dissociation on Rh(ll0) a nitrogen coverage of 0.75 ML has been suggested [22]. Fig. 7 shows the nitrogen uptake at 430 K where both Auger and TPD results were used. The data were normalized taking the integral of the Auger or TPD spectra corresponding to a saturated N coverage, Osat, as unity. From the TPD data in fig. 7 we estimated an initial sticking coefficient of 0.11 at 430 K, assuming that the saturation coverage is 0.5 ML. The adsorption

S. Lizzit et al. / Interaction

150

*

0.8-

l

l

ii 5 0.6 -

.

H F 0.4 (9

.

0.2 Onr*," 0

. ** 0.2

,,'I' 0.4

"'I 0.6

'I " 0.8

1

8 I &at VW Fig. 8. Plot of the N(KLL) Auger versus N, TPD normalized intensities.

rate declines gradually at nitrogen coverages above 0.1 of saturation. As illustrated in fig. 8, at lower NH, exposures the apparent coverage according to the Auger data is lower whereas the saturation of the N(KLL) Auger signal is achieved earlier as compared to the N, TPD data. Similar inconsistent between the intensity of the Auger and TPD spectra was observed during the isothermal experiments. Table 1 shows that compared to the corresponding TPD data the Auger intensities change more slowly at low reaction times and faster at high reaction times when the relative contribution of the broad peak at T > 550 K to the TPD integral increases. 3.3. LEED data The structure of N layers produced by dissociative NH, adsorption was found to be strongly dependent on the adsorption temperature. When the N layer was produced by NH, adsorption at T< 400 K the LEED patterns showed only an increased background and development of diffuse streaks in the [liO] direction. The corresponding N, TPD spectra had a pronounced peak below 500 K, as can be seen in figs. 1 and 3b. Adsorption of nitrogen via dissociation of N_H, at 430-450 K caused streaking along the [llOj azimuth at low coverages and subsequent development of streaky 3 x 1 and 2 X 1 structures with

of atomic

nitrogen with Rh(1 IO)

increasing nitrogen coverages as shown in fig. 7. Less streaky, with sharper h/3 spots, 3 x 1 patterns were produced during isotherma experiments via partial desorption of nitrogen from a saturated layer. The final 2 x 1 pattern also became less streaky when after exposure to NH, the system was kept at elevated temperature for a longer period or after a short flash to _ 490 K. This dependence of the sharpness of the patterns on the temperature indicates that the ordering of the N layer is an activated process. The intensity of the h/2 fractional order spots was comparable with that of the integer spots and did not change with increasing beam energy whereas the streaks became fainter at higher energies. Heating of a well developed 2 X 1 nitrogen overlayer led to the following changes. A stepwise increase of the temperature up to N 480 K caused sharpening of the h/2 fractional spots. During isothermal desorption at temperatures 48.5-515 K we observed development of new structures. Initially the 2 X 1 LEED pattern of a saturated N layer showed intense streaking along the [liO] direction. This was followed by a subsequent appearance of mixed 2 X 1-3 X 1, 3 X 1 and streaky 3 X 1 patterns, ending with a sharp 1 X 1 pattern after completion of nitrogen desorption. The LEED results obtained during isothermal desorption are presented in the last column of table 1. It shouId be noted that when the adsorption and isothermal desorption were performed at higher temperature the extra h/n spots were elongated and streaked exclusively in the [liOl azimuth, whereas in the case of lower adsorption and annealing temperatures the extra spots were more diffuse. This indicates that the long range order of the layer and the size of the ordered phase are temperature dependent. 3.4. N layer produced by Nz atomization on a hot W filament The experiments were performed using Nj” at pressures of (l-2) x lo-” mbar. Under these experimental conditions the nitrogen layer contained a substantial amount of coadsorbed CO and the use of labelled nitrogen was necessary in order to distinguish between nitrogen and CO

S. L&it et al. / Interaction of atomic nitrogen with Rh(ll0)

28

30 15 x7 I

400

500

600

700

800

900

1000 1100

Temperature (K) Fig. 9. N, (masses 30 and 1.5) and CO (mass 28) TPD spectra from a layer produced at 430 K by exposure to Ni5 atomized on a hot W filament. Heating rate: 2.5 K/s.

desorption. The Ni5 TPD spectra (m/e 30 and 15) are shown in fig. 9. Compared with the ones shown in fig. 2, it is quite obvious that the desorption of nitrogen occurs only in the temperature range of the broad high temperature feature. The desorption energy and the pre-exponential factor, evaluated from the leading edge, are 79 kJ/mol and 4 X lo4 s-l, respectively. From the TPD data the maximum nitrogen coverage is found to be about twice the saturation coverage achieved upon exposure to NH, at 430 K, whereas according to the Auger intensities it turns out to be about two times less. Such discrepancy is usual for systems where the adsorbate penetrates below the surface. Only very weak streaking along the [liOl direction was induced by this adsorption state of nitrogen. 4. Discussion 4.1. Stability of the nitrogen layer Comparison with the data reported for other fee (110) transition metal surfaces, such as Pd(ll0) [9] and Ni(llO)[10,12], where nitrogen desorption occurs within the temperature range of 800-900 K, the desorption temperatures of nitrogen observed in the present study indicates that on the Rh(ll0) surface the major amount of nitrogen is less strongly bound. The apparent first-order de-

151

pendence on the adatom coverage for associative desorption of the homonuclear diatomic N, molecule is not surprising. As discussed in ref. [23], the number of the nearest neighbour (NN) occupied site pairs which enters the rate equation becomes linearly dependent on the actual atomic coverage at high enough adatom coverages. Similar first-order dependence on the atomic coverage can persist at lower coverages as well when the adatoms tend to form islands of ordered structure containing NN site pairs. Our LEED results, where streaky 3 x 1 and 2 x 1 patterns were observed in a very wide coverage range, support the above explanation for a first-order reaction. At the lowest nitrogen coverages, when only very weak streaks along the [liO] azimuth were observed at low beam energies, the TPD traces resembled second-order rate process or were broadened because of the contribution of a growing higher temperature state. We tentatively ascribe these states to desorption from disordered and ordered phases coexisting on the surface. In the framework of the above general description the changes in LEED patterns and TPD spectra with increasing nitrogen coverage and after partial isothermal desorption (see figs. 2, 6 and 7 and table 1) can be interpreted as follows. At low coverages, where the fraction of nitrogen desorbing from ordered configurations is small (even negligible at the lowest coverages), the desorption parameters will be affected mainly by the energy variations within the disordered phase. With increasing nitrogen coverage and developing of ordered structures the species desorbing from the ordered phase with many next-nearest neighbours (NNNs) will be characterized by a higher desorption energy. It is a common trend that when the degree of order is enhanced the attractive NNN interactions are turned on and the stability of the adsorbed layer increases. The observed parallel changes of the pre-exponential factor obey the thermodynamic model of the transition state theory where In v N AE/kT. A similar compensation effect is very common and is observed with most of the adsorbate systems. The relation of the E, and V~ changes with nitrogen coverage to the structural changes occurring in the adlayer can also explain the higher

152

S. Lizzit et al. / Interaction of atomic nitrogen with Rh(l IO)

Ed and V~ values obtained for a nitrogen layer prepared by partial desorption at higher temperatures (see table 1). This can be simply ascribed to the existence of an activation barrier for the formation of the ordered phase, i.e., a better long range order is achieved after annealing at higher temperature, as evidenced by the increased sharpness of the extra spots. The values of the pre-exponential factor are relatively low if compared with the usual values expected for associative desorption (10”-1019 s-l) [24]. In the framework of the transition state theory these relatively low values indicate an immobile transition state. Another peculiar result is the lower E, value determined from the isothermal desorption experiments. This value is closer to the ones obtained at low coverages when the nitrogen layer is disordered. A possible explanation is that in the case of ordered layers the desorption parameters depend on whether desorption occurs at temperatures lower or higher than the critical temperature of the phase transition, T,. Using the leading edge method we evaluated Ed and ud for T < 500 K whereas the isothermal desorption experiments were performed at temperatures higher than 500 K. Assuming that in the latter case T zz T,, than the desorption parameters will describe desorption from a disordered rather than from an ordered phase. The broad N, desorption feature between 550 and 800 K is very similar to the one observed after dissociative adsorption of hydrazine on a Rh(ll1) surface [25]. There the recombination of the N adatoms occurred in the same temperature range and the authors proposed a second-order reaction limited recombination for description of the broad desorption spectra. In our case the 550-800 K state produced by exposure to NH, was a very small fraction of the desorbing nitrogen. Provided that its origin is similar to the one observed from a CO-contaminated nitrogen layer produced by a hot filament, no surface order can be related to this state. The most peculiar result is the very low values of the desorption parameters evaluated for this state, which cannot be justified assuming desorption via recombination of surface adatoms. As will be discussed in more detail in the section 4.2, this broad TPD feature

should be associated with desorption of nitrogen that has penetrated below the surface. Thus the desorption parameters evaluated for this state shouId be related to bulk-to-surface diffusion of nitrogen located in layers beneath the surface. In support of this suggestion is the weak N(KLL) Auger intensity corresponding to this state. 4.2. Structure of the nitrogen over-layer Comparison of the present LEED data with the corresponding TPD and Auger spectra should allow us to relate the LEED results to the actual N coverages. This knowledge is necessary to develop models for the observed surface structures, According to the calibration procedure described in section 3.2 the maximum coverage achieved under the present experimental conditions is supposed to be 2 0.5 ML. Provided that the saturation coverage is close to 0.5 ML then the h/2 fractional spots will achieve their maximum intensity at coverages close to saturation. On the contrary, inspection of the LEED-TPD results obtained both during adsorption and isothermal partial desorption (see fig. 7, and table 1) shows that the development of the 2 X 1 structure starts quite early (at coverages - 0.4 of saturation) and the intensity of the h/2 fractional spots reaches the highest sharpness and intensity at coverages - 0.7-0.8 of the saturation. Another inconsistency comes from the relation between the N coverage and the 3 x 1 structure which is best developed at coverages - 0.35 of saturation. Converted into monolayers this means that the N coverage corresponding to the 3 x 1 structure should be only - 0.18 ML, i.e., twice lower than the expected 0.33 ML. Foilowing the above considerations we suggest that the saturation coverage of nitrogen is larger than 0.5 ML. Than the N-induced 3 X 1 structure can be simply explained as due to ordering of the N adatoms. The periodicity is simply 3 and I along the [liO] and [OOll directions, respectively. There are several possibilities for adsorption sites of nitrogen so that we used the information from our previous HREELS study for similar nitrogen coverages in order to justify the location of nitrogen 1261. Our HREELS data showed that after

S. Ltizit et al. / Interaction of atomic nitrogen with Rh (110)

NH, adsorption on Rh(ll0) at 120 K and dissociation upon heating two vibrational modes at 26 and 54 meV remained in the spectra. Very similar vibrational modes in the range of 50-55 meV and at N 23 meV were reported for nitrogen adsorbed on Cu(ll0) 1251,Pd(ll0) [9] and Ni(ll0) [lo]. The loss peak at N 50-55 meV was interpreted as a metal--N mode with N adsorbed in a long bridge site, whereas the lower energy loss peak was ascribed to a surface phonon peak shifted to higher energies compared to the position of the same phonon of a N-free surface [9,10,27]. Consequently, we can describe the observed 3 x 1 patterns with a triple periodicity in the [liOl azimuth where nitrogen adatoms are in long bridge sites. A similar simple structural model also can explain satisfactorily the 2 x 1 LEED pattern. Assuming that the ordering is due to the nitrogen adatoms alone this means that only a fraction of the adsorbed species N 0.5 ML is involved in building of this ordered phase. Since as suggested above the saturation coverage exceeds 0.5 ML, then the 2 x 1 phase should coexist with N adsorbed in a disordered manner. The N adatoms forming the 2 x 1 structure can maintain the same long-bridge adsorption sites but it is unclear where the adatoms of the coexisting disordered phase are located. One possibility is that they are randomly distributed on the surface among the ordered adatoms. Then they will be responsible for the observed lowering of the desorption energy at eoverages close to saturation. Another alternative prompted by the observed discrepancy between the Auger and TPD data concerning nitrogen coverage is that a fraction of nitrogen is buried underneath the surface and can penetrate deeper Rh layers as well. The subsurface nitrogen can diffuse back to the surface during the desorption process when the surface sites are gradually liberated. This explains satisfactorily why at the initial stage of the isothermal desorption the Auger intensity decreases slower than the thermal desorption one. Since the energy barriers for processes such as subsurface and bulk penetration and back-diffusion are coverage dependent, the temperatures for formation and removal of the subsurface or of

153

nitrogen dissolved in the deeper layers will vary with the change in the relative population of the surface, subsurface and bulk regions. The general trend is an increase of the energy barriers for bulk penetration with increasing nitrogen subsurface concentration which is in good agreement with the obtained Auger results. However, the effect of the back diffusion on the TPD intensities is quite unpredictable because it depends on the depth of penetration, diffusion rate and actual distribution of the surface and subsurface concentrations. The broad N, peak at T > 550 K can be ascribed to segregation of nitrogen atoms from deeper layers but it is possible that some near-to-surface nitrogen contributes to the main features as well. That is why some inconsistency in the coverages evaluated on the basis of the TPD data is possible. The processes of bulk-tosurface and surface-to-bulk diffusion are very sensitive to the presence of traces of contaminants both on the surface or in the near-subsurface layers of the substrate. This is quite obvious from the N, TPD data obtained from nitrogen layers prepared by atomization on a hot filament where a substantial amount of CO is coadsorbed on the surface. The fact that we observed only a broad high temperature desorption indicates that similarly to the case of H + CO on Pd [28] the CO occupies the surface sites and pushes the N adatoms below the surface. Consequently, as mentioned above, the low Ed and v, values evaluated for this nitrogen state can be related to the kinetics of the bulk-surface diffusion process. Subsurface and bulk penetration of nitrogen has already been reported for Ni(l10) and Pd(ll0) surfaces [9,10] which in general exhibit an adsorptive behavior similar to Rh(ll0). Besides subsurface penetration N also induces distortion and reconstruction of these surfaces [9,10,29]. With the present experimental data we cannot exclude the possibility of a N-induced reconstruction of the Rh(ll0) surface as well. In fact, the following observations support a structural model involving N-induced reconstruction: (i) the independence of the h/2 fractional spot intensity on the beam energy and the sharpening of these spots at elevated temperatures; (ii) the existence of an ap-

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S. Lizzit et al. / Interaction of atomic nitrogen with Rh(ll0)

preciable activation barrier for formation of a well ordered layer; (iii) the sensitivity of the TPD peak width and position to the presence of traces of contaminants such as subsurface oxygen. The latter effect is related to the microscopic mechanism of reconstruction where besides the temperature and the adsorbate coverage the activation barrier for phase transition and the stability of the new phase can be affected by very small amounts of foreign atoms in the surface or subsurface region. According to the TPD data shown in fig. 3a the subsurface oxygen traces lead to an increase of the contribution of the more stable state at N 540 K which could be tentatively associated with the reconstructed phase. The N-induced reconstruction of the Rh(ll0) surface could be similar to the O-induced 2 x 1 reconstruction of Cu(ll0) and Ni(ll0) surfaces where 0 occupies long bridge sites and every second [loo] row is missing [30,31]. The streaking in the [liO] direction would then indicate that the layer consists of anisotropic islands whose dimensions are smaller in the [liO] direction. For such a rather open reconstructed surface nitrogen subsurface penetration and diffusion to deeper layers will be more facile. Another restructuring alternative is buckling of every second Rh atom from the [liO] row due to incorporation of N below the surface. However, these are only suggestions and more in-depth structural studies are necessary to determine whether a reconstruction occurs.

5. Conclusions The analysis of the TPD, LEED and Auger data shows the following quite complex behaviour of the N/Rh(llO) system: (1) The N, TPD spectra due to associative desorption exhibit an unusual shift towards higher temperatures with increasing N coverage due to development of more stable adsorption states. The overall shift and the width of the TPD spectra are determined by the relative contribution of these states and are extremely sensitive to the presence of traces of subsurface oxygen. (2) The changes in the TPD spectra are accompanied by formation of ordered 3 X 1 and

2 X 1 structures. The fractional h/2 spots are as intense as the substrate spots up to the onset of desorption. The ordering of the layer is an activated process and is favoured by elevated temperatures. (3) There is a discrepancy between the observed changes in the N(KLL) and N, TPD intensities used as a measure of the N coverage. This indicates that a fraction of N atoms is buried underneath the surface. The stabilization of the N layer with increasing N coverage and the ordering of the layer might involve N-induced restructuring of the surface. This would facilitate penetration of a fraction of nitrogen below the surface.

Acknowledgement

Technical assistance by M. Barnaba Sandrin is gratefully acknowledged.

and G.

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