oxygen states at magnesium and copper surfaces revealed by scanning tunneling microscopy and surface...

Oxygen states at magnesium and copper surfaces revealed by scanning

tunneling microscopy and surface reactivity

A.F. Carley, P.R. Davies, K.R. Harikumar, R.V. Jones, and M.W. Roberts�

Department of Chemistry, Cardiff University, P.O. Box 912, Cardiff CF10 3TB, UK

By a combination of STM and XPS a study of the dynamics of oxygen chemisorption at Mg(0001) at 295K has revealed oxygen

states involving nucleation sites and the development of hexagonal and square lattice structures; the hexagonal structures develop

epitaxially with the Mg(0001) surface. There is extensive surface mobility with, at the early stage of chemisorption, oxygen states

being observed at steps at the ð1� 1Þ-O adlayer and overlapping magnesium atoms. These are active in ammonia and hydrocarbon

oxidation whereas at higher oxygen coverage the surface is inactive. The dissociative chemisorption of nitric oxide generates a

surface characterized by the hexagonal oxide structure; nitrogen adatoms known to be present from the N(Is) spectra are

disordered. The chemisorption of hydrogen chloride at a Cu(110) surface results in a cð2� 2Þ–Cl structure with at high coverage

domains 1.8 nm wide. A sub-surface oxygen state at Cu(110), although unreactive to ammonia, undergoes a chemisorption

replacement (corrosive chemisorption) reaction with HCl at 295K.

KEY WORDS: magnesium; copper; scanning tunneling microscopy; X-ray photoelectron spectroscopy; ammonia; oxidation;

hydrocarbons.

1. Introduction

It is appropriate that in this paper, to commemorateJohn Meurig Thomas’ 70th birthday, we discuss oxygenstates at metal surfaces a theme that is very close to thatwhich John investigated as part of his PhD thesis. Thisconcerned the oxidation of ‘‘clean’’ carbon surfaces, atopic that he pursued subsequently with much vigor andtenacity [1,2] when he was appointed in 1959 to alectureship at the University College of Wales atBangor, making use initially of optical microscopy,later electron microscopy and X-ray photoelectronspectroscopy (XPS). In one sense this paper mirrorsthat strategy in that it combines chemical informationfrom electron spectroscopy with structural informationat the atom resolved level, from scanning tunnelingmicroscopy (STM).

Oxygen states at metal surfaces have been a recurrentinterest of one of us for over 30 years. Initially usingwork function studies [3] to establish the relative easewith which oxygen incorporation occurred at lowtemperatures (100–150K) and the defective nature ofthe chemisorbed overlayer [4]. Electron energy distribu-tion (photoemission) data provided further confirmationof oxide formation and significantly that the photoelec-tron escape depth was no more than 1 nm [5]. It was thelatter that provided the stimulus for the development ofan ultra-high-vacuum photoelectron spectrometer as ameans of exploring the defective oxygen states at nickeland nickel oxide surfaces through monitoring shifts in

core-level binding energies [6]. The end game, however,was to understand chemical reactivity and surfacecatalysis and particularly the role of defect oxygenstates present, albeit transiently, during the formation ofthe chemisorbed adlayer. Low energy electron diffrac-tion (LEED) in conjunction with optical simulationstudies [8] also provided evidence for oxygen adatommobility in the formation of a defective chemisorbedadlayer at W(211) and Cu(210) surfaces and it was theseconclusions that led us to use probe molecules incoadsorption studies with oxygen, using surface spectro-scopies and STM. The Mg(0001) surface was chosen, forstrategic reasons, as a model system for a concertedstudy with both ammonia [7a] and propene [7b] used as‘‘probe molecules’’. The overriding conclusion fromXPS studies [7] was that reactive metastable oxygen O�

states existed during the formation of the chemisorbedoverlayer which were highly reactive in oxidative de-hydrogenation whereas the chemisorbed ‘‘oxide’’ over-layer was unreactive. The O�

ðsÞ transients were gener-ated as intermediates in the dissociative chemisorptionof dioxygen, nitric oxide and nitrous oxide. This hadimplications not only for a fundamental understandingof the individual steps involved in surface catalysis butalso for the dynamics of oxygen chemisorption involvinghot oxygen atoms [7].

One of the outstanding aspects to emerge from STMstudies has been the atom resolved evidence for themobility of surface atoms at low temperatures and inthis paper we explore how STM, with the advantage ofin situ XPS, has provided a further insight into theoxidation of Mg(0001) and following on from ourearlier studies of the Cuð110Þ–O system [9], elucidatingconditions (surface coverage and temperature) for which

�To whom correspondence should be addressed.

E-mail [email protected]

Topics in Catalysis Vol. 24, Nos. 1–4, October 2003 (# 2003) 51

1022-5528/03/1000–0051/0 # 2003 Plenum Publishing Corporation

chemisorbed oxygen becomes unstable with respect to asubsurface state.

2. Experimental

XPS and STM data were acquired using a combinedSTM/XPS instrument supplied by Omicron VacuumPhysik. The sample was sputtered clean using argon ionsat an energy of 0.6 kV and subsequently annealed at�750K for 45min for the Cu(110) sample and 475K for60min for Mg(0001). Surface cleanliness was monitoredby XPS and the purity of the gases, (hydrogen chloride(99.9%) and ammonia (99.8%), supplied by BDHInternational Ltd.; nitrous oxide (99.5%) and nitricoxide (99.0%) oxygen (99.998%) supplied by Argo GasesLtd.), monitored with in situ mass spectrometry. Gasdosing was performed with the ion gauge on but thegauge was some distance from the sample and noactivated adsorption caused by the filament wasobserved in control experiments. All photoelectronspectra were recorded with an AlK� photon sourceusing a pass energy of 50 eV and calibrated against theCuð2p3=2Þ peak at 932.7 eV and the Mg(1s) peak at1303 eV. Spectra were acquired using commercial soft-ware (SPECTRA; R. Unwin) and analysed usingsoftware developed in-house.

The calculation of the surface concentration ofadsorbates from photoelectron data was discussed indetail by Carley and Roberts [10], the subshell photo-ionization cross sections tabulated by Scofield [11] beingused in equation (1), a modification of that proposed byMadey et al. [12]:

�a ¼Ia�sEaN�� cos�

Is�aEsMð1Þ

where: �a ¼ concentration of the adsorbate; Ia=Is ¼integrated adsorbate and substrate line intensities;�a=�s ¼ modified [13] Scofield photoionisation crosssections, � ¼ density of substrate, Ea=Es ¼ kineticenergy of photoelectrons, � ¼ mean free path ofelectrons within substrate, � ¼ angle of collection withrespect to the sample normal; M ¼ relative atomic massof the substrate, N ¼ Avogadro’s constant. The tung-sten STM tips were electrochemically etched in KOHsolution at a voltage of approximately 10V. TypicalSTM scanning conditions for the Cu(110) and Mg(0001)surfaces were similar, with a tunneling current between2.9 and 3.5 nA and a sample bias of �1:2V.

3. Oxidation states at Mg(0001)

Structural studies of oxygen chemisorption atMg(0001) surfaces were reported by Namba et al. [14]and Hayden et al. [15] using low energy electrondiffraction (LEED), work function and Auger electron

spectroscopy. The insensitivity of LEED to surfacedisorder was emphasized by Namba et al. who alsoexpressed reservations regarding the model they hadproposed. Earlier kinetic studies by Rhodin [16] hadsuggested that a nucleation process was involved at theearly stages of oxygen chemisorption, the rate increasingwith coverage. This is an aspect that STM, with atomicresolution, has an advantage over both LEED and otherless direct methods.

In figures 1 and 2 are shown a series of sequentialSTM images observed during the dissociative chemi-sorption of N2O at a Mg(0001) surface at 295K. Theimages show the development of isolated hexagonalstructures, which are typically 3 A high. Most islands weobserved were stable but occasional examples, such asthat shown in figure 2, establish that these are bilayerstructures with the upper layer undergoing translationalmotion relative to the lower layer across the Mg(0001)surface which, at this stage, is in the main atomicallyclean. Line profiles of these structures indicate a stepheight of between 0.14 and 0.15 nm for the overlappingMgð0001Þ–O–ðMgÞ bilayer (figure 3). We have presentedelsewhere [17] a model for these structures whichinvolves an upper magnesium layer and a lower 1� 1epitaxial oxygen layer and we have discussed also thesignificance in terms of the chemical reactivity of specialchemisorbed oxygen states present at the step.

With increasing oxygen exposure, and surface cover-age, both hexagonal and square lattice structuresdevelop (figures 1(d) and 4). The hexagonal structureis most prominent with the square lattice (110) typestructure being a minor component of the ‘‘oxide’’overlayer. The repeat spacings in the hexagonal andsquare lattices are 0.321 and 0.56 nm, respectively (figure4). The spacing in the hexagonal structure is virtuallyidentical with the Mg–Mg interatomic spacing in theclean Mg(0001) surface suggesting that the chemisorbedoxygen develops as a ð1� 1Þ eptiaxial layer. The squarelattice component has a structural resemblance to thecubic MgO ‘‘smoke’’ formed when magnesium vapor isoxidized at high temperatures [18]. The XP spectrumindicated that the final chemisorbed state (figure 4) wascharacterized by an O(1s) peak at a binding energy of530.5 eV, analysis of its intensity indicating the presenceof 2� 1015 oxygen adatoms cm�2. At this stage thesurface plasmon peaks associated with the Mg(1s) peakand characteristic of the clean metal, were of very lowintensity confirming that the surface was ‘‘oxidized’’.

These STM observations contrast with those ofGoonewardene et al. [19] who at no stage provideatom resolved evidence for the nature of the chemi-sorbed oxygen layer at Mg(0001). These authors reporttip induced movement of surface magnesium atoms butwhich with increasing oxygen exposure stabilize. At anoxygen exposure of 4L the surface develops ‘‘bumps’’ orpillars which have a base diameter of 3.5 nm and aheight of 0.36 nm. We do, however, observe features

A.F. Carley et al./Oxygen states at magnesium and copper surfaces revealed by scanning tunneling microscopy and surface reactivity52

somewhat similar to those reported by Goonewardeneet al. [19] when we expose the surface to oxygen whilesimultaneously scanning close to the surface (40 nm orless). When the STM tip is moved elsewhere there are nosuch features and the surface appears to be atomicallyflat. When N2O is used for surface oxidation studies notip influence is observed and the images are identicalwith those observed when O2 is used (figure 4) as theoxidant, but without simultaneously scanning the sur-face.

4. Ammonia oxidation at Mg(0001)

Coadsorption of an ammonia-rich NH3–N2O (15 : 1)mixture at a Mg(0001) surface at 295K resulted in theformation of a hexagonal structure identical to that

shown in figure 4. The total exposure was 1000Lð1L ¼ 10�6 torr sÞ and analysis of the O(1s) and N(1s)intensities at 530.5 and 399 eV, respectively indicatedsurface concentrations of 5:3� 1014 oxygen adatomscm�2 and 2:6� 1014 nitrogen species cm�2. The atom-resolved hexagonal structure is associated with theepitaxial chemisorbed oxygen ð1� 1Þ adlayer, the lineprofile indicating a spacing of 0.321 nm, matchingclosely the Mg–Mg interatomic distance in Mg(0001),with a height of 0.125 nm between two separatehexagonal islands. These height differences are variableand can be as much as 0.6 nm in some cases.

No structural features that could be attributed tonitrogen species were observed in the STM images. TheN(1s) binding energy value of 399 eV indicates that thenitrogen is present as NH2 (a) species, in agreement withour earlier studies [7].

Figure 1. A series (a,b,c) of STM images taken 90 s apart, of the nucleation and growth of a hexagonal ð2� 1ÞO structure formed by the

dissociative chemisorption of N2O at Mg(0001) surface at 295K; (d) scan of the Mg(0001) surface after exposure to 72L N2O; O(1s) spectra

indicate an oxygen concentration of 7:7� 1014 cm�2 at this stage.

A.F. Carley et al./Oxygen states at magnesium and copper surfaces revealed by scanning tunneling microscopy and surface reactivity 53

Figure 2. A series of STM images (a–f) taken 90 s apart indicating the mobility of the hexagonal structures at 295K; in image (b) the hexagonal

nucleus has separated to reveal a bilayer structure.

Figure 3. (a) A high resolution image of the bilayer structure revealing a ð2� 1ÞO chemisorbed layer present at a step and overlapped by

magnesium surface atoms; note the special O�� sites at the step; (b) line profile indicating a step height of 0.15 nm; (c) structure of the bilayer

surface structure.


5. Chemisorption of nitric oxide at Mg(0001)

Nitric oxide is known from earlier XPS studies [20] tobe dissociatively chemisorbed at Mg(0001) at 295K.Intense O(1s) and N(1s) peaks were observed at bindingenergies of 530.5 and 395.3 eV respectively, analysis ofwhich indicated the presence of 1:7� 1015 oxygenadatoms cm�2 and 9:7� 1014 nitrogen adatoms cm�2.STM images indicate that the adlayer consists of bothordered and disordered structures (figure 5). Annealingto 470K did not lead to an ordering of the disorderedregions. In a separate experiment we investigatednitrogen adatoms implanted into the magnesium surfacethrough an ion gun. This also resulted in an XPS peak at395.3 eV and a disordered surface which was unaffectedby extensive annealing. We, therefore, assume that thedisordered regions observed (figure 5) are associated

with nitrogen adatoms and the N(1s) peak at 395.3 eV.This contrasts markedly with the well ordered ‘‘nitro-gen’’ structure present either as NH(a) or N(a) atCu(110) surfaces [9].

6. Oxygen chemisorption at Cu(110); a subsurface state

Five different oxygen states have been recognized bySTM to be present at a Cu(110) surface [9], in thetemperature range 60 to 295K. We explore here theoxygen states present up to 550K and in particular thestability of chemisorbed oxygen with respect to itsincorporation to a subsurface state.

An atomically clean Cu(110) surface was exposed(10L) to oxygen at 295K resulting in the formation of acomplete ð2� 1ÞO adlayer (figure 6(a)). Analysis of the

Figure 4. With increasing oxygen exposure the Mg(0001) surface consists of both hexagonal and square lattice structures at 295K; line profiles

indicate that the repeat distance in the atom resolved hexagonal structure is 0.321 nm and in the square structure 0.56 nm; the hexagonal structure

is the dominant one.


corresponding O(1s) spectrum which has a bindingenergy of 529.7 eV (figure 6(a)) indicates that the con-centration of chemisorbed oxygen is 5:1� 1014 cm�2.After heating this adlayer to 550K in vacuum both theSTM image and the O(1s) spectrum were unchangedindicating the thermal stability of the ð2� 1ÞO mono-layer (figure (b)).

In an analogous experiment a partially covered oxygenadlayer, formed by exposing the Cu(110) surface to oxygen(3L) at 295K, was heated to 550K. Figure 7 shows theO(1s) spectra and STM images observed at both 295K andafter heating to 550K. The Cu(110) surface is partiallycovered with ð2� 1ÞO islands at 295K and the O(1s)binding energy is at 529.7 eV; the surface oxygen atomconcentration is 2:3� 1014 cm�2. After heating to 550K

the O(1s) intensity decreased slightly, with a perceptibleshift to higher binding energy (530 eV). The calculatedoxygen concentration is 2:0� 1014 cm�2 but significantlythe STM image is of a clean Cu(110) surface (figure 7(b)).

From these experiments it is clear that although amonolayer of chemisorbed oxygen is stable at a Cu(110)surface up to 550K the sub-monolayer ð� ¼ 0:4Þ istransformed to a sub-surface state on heating to 550K.This is further confirmed by exposing the latter surfaceto oxygen at 295K when the STM image (figure 7(c))shows a completed ð2� 1ÞO structure and an O(1s) peakat 529.7 eV, analysis of which indicates an oxygenconcentration of 6:3� 1014 cm�2 (figure 7(c)). This issubstantially greater than that present in the completeð2� 1ÞO chemisorbed layer ð5:5� 1014 cm�2Þ.

Figure 5. STM images (a) of the disordered and (b) ordered surface regions present at a Mg(0001) surface which has dissociatively chemisorbed

nitric oxide at 295K; (c) an example of a highly-faceted structure on heating to 430K.

Figure 6. XP spectra and STM images for a Cu(110) surface (a) after exposure to 10L oxygen at 295K; (b) after heating to 550K in vacuum for

1 h and cooling to 295K. The oxygen concentrations are as shown; note the thermal stability of the ð2� 1ÞO layer.


7. Chemical reactivity of oxygen states: the oxidation of

ammonia and corrosive chemisorption of hydrogen

chloride

A Cu(110) surface exposed to oxygen (3L) at 295Kgenerated a surface partially covered with oxygenadatoms which on heating to 550K transformed to asubsurface state. The O(1s) binding energy was at 530 eVand the calculated oxygen concentration was4:1� 1014 cm�2 (figure 8(b)). When the subsurfaceoxygen state was exposed to an ammonia-dioxygen(30 : 1) mixture at 450K, the O(1s) peak intensity andbinding energy remained unchanged but an intenseN(1s) peak developed at 396.5 eV (figure 8(c)). This ischaracteristic of chemisorbed nitrogen adatoms [21] andthe STM image (figure 8(d)) established that they werepresent as a ð2� 3ÞN structure. Analysis of the N(1s)intensity indicated that the nitrogen adatom concentra-tion was 6:3� 1014 cm�2, i.e., close to that expected of afully-formed chemisorbed layer. There is no evidence inthe STM image for any chemisorbed oxygen beingpresent which would have been recognized by itscharacteristic ð2� 1Þ structure running along the h001idirection, i.e., at right angles to the ð2� 3ÞN structure(figure 8(d)). It is clear that the reactivity of the surfacecopper atoms is not modified by the presence ofsubsurface oxygen. This was further confirmed whenthe subsurface oxygen state was exposed to just

ammonia at 550K, no N(1s) intensity was generatedand the O(1s) intensity was essentially unchanged.

In view of our earlier XPS studies of the interactionof HCl(g) with Cu(111)–O surfaces, leading to achemisorptive replacement reaction to give a Cu(111)–Cl adlayer at low temperatures [22], we investigated HClchemisorption at both Cu(110) and Cu(110)–O surfaces.At the clean Cu(110) surface a flat cð2� 2Þ chlorineadlayer forms (figure 9(a)). Chemical information (XPS)and structural evidence (STM) established that incontrast to ammonia, subsurface oxygen is reactive toHCl(g) being chemisorptively replaced by chlorine withsimultaneous desorption of water at 295K detected bymass spectrometry. The structural features observed bySTM (figure 9(b)) are generally poorly resolved but runalong the h001i direction and show strong similarities tothose observed when chemisorbed (surface) oxygenpresent at a Cu(110)–O surface is exposed to HCl(g).In figure 9(c) and (d) are shown two STM images takensequentially showing the development of two suchstructures (marked A) pinned between two step edgesat a Cu(110) surface. It is clear that these develop as aresult of nucleation (reaction) at the end of theelongated structures. In the presence of oxygen, whethersubsurface or at the surface this is a highly exothermiccorrosive-type reaction leading to increased surfacediffusion and the formation of the furrows runningalong the h001i direction. That enhanced surfacediffusion of copper occurred in the presence of chlorinewas first emphasized by Delamare and Rhead [23] whosuggested that the surface simulated a ‘‘quasi twodimensional liquid’’ at 600K.

8. Conclusions

Three distinct oxygen states have been recognized bySTM to develop during oxygen chemisorption (whetherfrom O2 or N2O) at a Mg(0001) surface at 295K.Hexagonal bilayer structures are predominant in whichthe epitaxial oxygen adlayer is subsurface. However,during initial nucleation and island growth a secondoxygen state is revealed through the lateral translationof the upper magnesium atoms. Finally a square latticestructure (analogous to MgO ‘‘smoke’’) develops as aminor component of the surface topography.

What is particularly significant is that it is during theearly stages of oxygen chemisorption, i.e., at thenucleation stage, when there is extensive surfacemobility, and leading to the formation of the hexagonaland square lattice ‘‘oxide’’ structures, that the Mg(0001)surface is catalytically active in the oxydehyrogenationof ammonia [7a] and the oxidation of propene [7b] togive benzene and a C4 hydrocarbon probably butadiene.Radical reactions were implicated with O� transientspresent as the reactive species prior to the surfacebecoming inactive when Mg(1s) spectra indicated that

Figure 7. XP spectra and STM images (a) for a partially oxygen

covered ð2� 1Þ adlayer at a Cu(110) surface at 295K; (b) after heating

in vacuum to 550K and (c) after further oxygen exposure at 295K.

Note the absence of any surface ð2� 1ÞO structure in (b) even though

XPS indicates a concentration of 2:0� 1014 oxygen atoms cm�2.


Figure 8. (a) O(1s) spectrum for a clean Cu(110) surface; (b) after exposure (3L) to oxygen at 295K followed by heating to 550K in vacuum and

cooling to 295K; also shown is the N(1s) spectral region; (c) followed by exposure to a NH3 : O2 (30 : 1) mixture at 450K for which both O(1s)

and N(1s) spectra and the STM image observed at 295K are shown. Oxygen and nitrogen adatom coverages at each stage (b) and (c) also

indicated. Note the ð2� 3ÞN structure in the STM image.

Figure 9. STM images for (a) the cð2� 2ÞCl structure at a Cu(110) surface after chemisorption of HCl at 295K; (b) elongated structures aligned

along the h001i direction after exposure (50L) of the subsurface oxygen state at Cu(110) to HCl at 295K; (c) and (d) the nucleation and growth of

the elongated structures pinned at steps in the Cu(110) surface.


oxidation (the presence of Mg2þ) had occurred. The roleof oxygen transients in controlling reaction pathwayshas been reviewed recently [24].

At Cu(110) surfaces the conditions under which asubsurface oxygen state can exist in the absence ofchemisorbed surface oxygen have been delineatedemphasizing the advantage of combining STM withXPS. Subsurface oxygen, although unreactive to ammo-nia, is reactive to hydrogen chloride with chemisorptivereplacement of oxygen and the formation of surfacechlorine adatoms with characteristic features observedby STM. At a Cu(110) surface hydrogen chloridechemisorption results in a cð2� 2Þ–Cl structure withevidence of surface buckling with domains some 1.8 nmwide.

Acknowledgment

We are grateful to EPSRC for their support of thiswork.

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