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Surface Science 602 (2008) 499–505

Silver nanostructures on a c(4 · 2)-NiOx/Pd(100) monolayer

S. Agnoli a, M. Sambi a, G.A. Rizzi a, G. Parteder b, S. Surnev b,F.P. Netzer b, G. Granozzi a,*

a Dipartimento di Scienze Chimiche, INSTM and INFM Research Unit, Universita di Padova, Padova, Italyb Institut fur Physik, Oberflachen-und Grenzflachenphysik, Karl-Franzens Universitat Graz, A-8010 Graz, Austria

Received 25 September 2007; accepted for publication 31 October 2007Available online 7 November 2007

Abstract

The growth, morphology and epitaxial relationship of Ag nanostructures deposited onto the c(4 · 2)-NiOx/Pd(100) surface have beeninvestigated by photoemission (both core and valence levels), scanning tunneling microscopy and angle-scanned photoelectron diffrac-tion. Small Ag nanoparticles are obtained on the terraces, whereas the tendency of Ag to decorate the step edges can lead to the forma-tion of extended nanowire-like features. The Ag nanoparticles adopt a fcc structure epitaxially related to the substrate according to thefollowing relationship: Ag(10 0)[001]//Pd(100)[001], while by means of STM we have found that the shape of the islands is typicallyrectangular with the edges running along the [011] directions. According to the reported data, the NiOx ML is locally disrupted inthe vicinity of the Ag nanoparticles so that the c(4 · 2)-NiOx/Pd(1 00) surface cannot be considered as an efficient template to stabilizemetal nanoparticles against Ostwald ripening phenomena.� 2007 Elsevier B.V. All rights reserved.

Keywords: Ag nanoclusters; NiO/Pd(100) monolayer; STM; Photoemission; Photoelectron diffraction

1. Introduction

Metal nanostructures show a large variety of interestingproperties finding applications in many different fields,such as microelectronics and photonics [1], energetics [2],magnetism [3] and catalysis [4].

These properties are in general critically dependent onthe structure of the nanoparticles at the atomic level.Therefore, many efforts have been undertaken to developefficient synthetic routes to accurately control the sizeand the morphology of metal nanostructures [5]. Nanofab-rication, i.e. the control of the shape, dimensions and orderof such nanostructures, can be achieved with the well-known sequential top-down approach, e.g. by nanolitho-graphic methods, or by atomic manipulation via ScanningTunneling Microscopy (STM) [6]. Alternatively, size se-lected cluster deposition methods can be used, even if clus-

0039-6028/$ - see front matter � 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.susc.2007.10.047

* Corresponding author. Tel.: +39 049 8275158; fax: +39 049 8275161.E-mail address: gaetano.granozzi@unipd.it (G. Granozzi).

ter ordering is difficult to achieve [7]. However, the currentfrontier in nanofabrication is associated with the formationof nanostructures by a fast parallel self-assembly process.One of the most promising techniques is the use of a suit-able template surface (e.g. an oxide ultrathin film with anordered array of defects which act as preferential nucle-ation centres), which is able to orient the subsequentgrowth of metal particles leading to the desired surfacenanostructures [8–10].

Ag based nanostructures, in particular, play an impor-tant role in nanotechnology since this material is very suit-able for the development of basic device elements, such aselectrical devices with quantum conductance [11] or plas-mon based optical elements [12]. Moreover, Ag nanoparti-cles (NP) and nanowires are also widely used as catalysts[13] and are at the forefront of the new single molecule sen-sors [14,15]. For these reasons, the study of surface sup-ported Ag NPs is widely addressed a topic in theliterature: a large amount of papers considering widely dif-ferent substrates, ranging from semiconductors [16,17] to

500 S. Agnoli et al. / Surface Science 602 (2008) 499–505

oxides [18–20], from flat to vicinal surfaces [21,22], havebeen published. Also, Ag on NiO is of relevance becauseAg is used as a promoter in NiO based catalysts [23,24]or as an additive to tailor the optical properties in electro-chromic devices [25].

In recent years, the reversed catalyst model system i.e.the growth of NiO on Ag(100), has been thoroughly stud-ied and the NiO/Ag(100) interface has assumed the statusof a reference system for a well-matched ultrathin film [26–32].

We have reported previously the structure and morphol-ogy of NiO/Pd(100) ultrathin films, with particular atten-tion towards novel structures and properties stemmingfrom the large lattice mismatch [33–37]. In the monolayer(ML) coverage range a new defective NiOx c(4 · 2) struc-ture (x = 1.33) has been thoroughly investigated by bothstatic and dynamic low energy electron diffraction (LEED)[36], STM [37], and photoemission spectroscopy (both va-lence and core levels) [38]. In the following we present amorphological and chemical characterization of the Ag/NiOx/Pd(1 00) system, prepared by Ag UHV evaporationon the c(4 · 2) NiOx/Pd(1 00) ML film, for different Agcoverages. The choice of the investigated system was dic-tated by the goal of testing if the defective c(4 · 2) ML issufficiently stable to act as an effective template for thegrowth an ordered array of Ag NPs, as demonstrated inother cases [8–10].

2. Experimental

The experiments have been performed in two differentUHV systems. The first one, used for the photoemissionexperiments (both core, XPS, and valence band, UPS, mea-surements), is a two-chamber UHV system (a modified VGESCALAB MK II, Vacuum Generators, Hastings, Eng-land), equipped with a four grids rear view LEED, twoelectron beam evaporators with an integrated flux monitor,a quadrupole mass spectrometer, a twin (Mg/Al) anode X-ray source, a discharge lamp for noble gas ionization (VUVHIS 13 Omicron), a sputter gun, and a hemispherical elec-trostatic analyzer ending with a five channeltrons detector.The sample is mounted on a two-axis goniometer capableof sample rotations in polar angle h (defined with respectto the surface normal) and in azimuthal angle /, (definedwith respect to the [001] direction of the substrate surface),thereby allowing us to collect angle-scanned X-ray photo-electron diffraction (XPD) data. Angular accuracy is al-ways better than ±1� in both directions. The angularacceptance of the analyzer can be varied between 1.5�and 8� (the latter used for UPS experiments). The bindingenergy (BE) calibration was performed using the FermiEdge (EF) and 4f peaks of a gold sample.

The STM experiments were performed in Graz, in aUHV system equipped with a variable-temperature STM(Oxford Instruments), LEED, an Auger electron spectrom-eter (AES), and facilities for crystal cleaning and metalevaporation (Omicron EMF3 triple electron beam evapo-

rator). The STM images were recorded in constant currentmode at RT, with electrochemically etched W tips, whichhave been cleaned in situ by electron bombardment. Typi-cal tunneling conditions employed for imaging were 1.5 Vand 0.1 nA.

For all the experiments the Pd crystal was cleaned by re-peated cycles of argon ion sputtering (E = 2 keV), andannealing in 1 · 10�6 mbar O2 (uncorrected ion gaugereading) and a final flash to T = 970 K. The cleanlinessof the substrate surface prior to the experiments was alwayschecked by using XPS or AES and the surface order wasprobed by means of LEED.

In order to grow the c(4 · 2)-NiOx structure the stan-dard recipe was followed, described in details in Ref. [34].Silver was evaporated using an e-beam evaporator underUHV conditions at room temperature (RT).

The evaporation rate of both Ni and Ag was determinedby angle resolved XPS (ARXPS) in the photoemissionstudies or by a quartz microbalance calibration in theSTM experiments. The deposition rates employed for Niand Ag were �0.5 MLE/min and �0.2 MLE/min, respec-tively. One monolayer equivalent (MLE) is referred tothe atom density of the Pd(100) surface and correspondsto 1.3 · 1015 metal atoms/cm2.

3. Results and discussion

In the present study, we have investigated the chemical,morphological and structural evolution of the Ag/NiOx(1ML)/Pd(10 0) system for increasing amounts of Ag. Ineach experiment, the initial surface was prepared by reac-tively depositing 0.75 MLE NiO ðP O2

¼ 10�6 mbarÞ onPd(100) at RT and successively annealing the sample at250 �C in 5 · 10�7 mbar O2. Under these conditions a flatcontinuous layer of c(4 · 2)-NiOx is formed with very fewvacancy islands. For a detailed description of this surfacewe refer to our previous work [34–37].

3.1. XPS results

In Fig. 1 we report the photoemission data of the Ag/NiOx/Pd(100) system as a function of the Ag coverage.All core level spectra were taken at grazing incidence(h = 70� from the surface normal) with an Al Ka non-monochromatized source. Fig. 1a reports the XPS Ni 2p.For a detailed interpretation of the Ni 2p photoemissionspectrum of the c(4 · 2) phase we refer the reader to a pre-vious publication [38]. Here it is sufficient to note that assoon as Ag is deposited, a shoulder appears at the higherBE side of the main peak, and the peak maximum progres-sively shifts towards higher BE while increasing the Ag dose(at 2.5 MLE DEBE = 0.8 eV). At 2.5 MLE a single peak isobservable, whose full width at half maximum (FWHM)is slightly decreased (by 0.2 eV) with respect to thec(4 · 2) ML. The structure and the intensity of the satellite,on the other hand, do not undergo significant changes afterthe Ag deposition. Fig. 1b shows the XPS spectrum of the

Fig. 1. Photoemission data of the Ag/NiOx/Pd(100) system as a function of the Ag coverage: (a) Ni 2p XPS region (AlKa source), (b) Ag 3d region(AlKa source) and (inset) Ag MNN Auger peak for the 0.3 MLE deposit, (c) VB region (He-II source).

S. Agnoli et al. / Surface Science 602 (2008) 499–505 501

Ag 3d photoemission line. It can be clearly seen that theBE maximum of the Ag 3d5/2 component shifts progres-sively towards higher BE as a function of coverage (DEBE =0.3 eV), while the FWHM remains constant. At 2.5 MLEthe BE matches that of an Ag(100) bulk-like sample. In or-der to check the oxidation state of Ag on the surface, wehave measured the Ag MNN peak (see the inset in Fig. 1bin the case of a 0.3 MLE deposit) to evaluate the Augerparameter. It turns out that this it changes from 720.1 eVin the case of the 0.15 MLE to 720.0 eV in the case of the2.5 MLE, i.e. a difference which is not significant withinthe experimental error, while both values are compatiblewith a pure metallic state. The XPS region containing thePd 3p3/2 and O 1s peaks is rather uninformative, due tothe large overlap between the two peaks. Anyway, no signif-icant change in the band envelope as a function of the Agcoverage is observed and the peak positions are those al-ready reported for the substrate (Pd 3p3/2 component at531.9 eV, O 1s component at 529.3 eV in NiOx/Pd(100))[38]. Fig. 1c shows the He II excited VB spectra taken atnormal emission (Cpoint) as a function of the Ag coverage.The most relevant trend is the progressive formation of twodistinct peaks in the region between 3 eV and 7 eV which areclearly due to the progressive formation of the Ag 4d band.Just below EF (region between 0 and 3 eV), the changes areless distinct, but one can see that the features peculiar of thec(4 · 2)-NiOx/Pd(100) system [38] become broader and lessintense because of the attenuating effect of the Ag overlayerand of the overlap with its 5sp band.

3.2. STM results

The morphological changes resulting from the Ag depo-sition have been investigated by STM. After the deposition

of 0.15 MLE Ag on the c(4 · 2) NiOx/Pd(100) phase(Fig. 2a), several irregular Ag NPs can be seen on the sur-face. In particular, they preferentially decorate the (zigzagreconstructed) step edges caused by the NiOx layer. TheNPs have a size dispersion ranging between 20 A and50 A, and can be single (apparent height with respect tothe NiOx ML � 1.8 A) or double layered (3.5 A). The insetof Fig. 2a shows a high resolution image of an Ag NP onthe c(4 · 2) superstructure: as it can be seen, the oxide layeris partially disrupted in the close proximity to the Ag NP.Increasing the coverage to 0.3 MLE (Fig. 2b) leads to astronger decoration of the step edges, which straightenout and become decorated by long nanowire-like featuresrunning continuously for several hundred A. Also, islandslocalized on the terraces become more frequent and tend toassume better developed rectangular shapes. In addition, itis possible to distinguish the formation of a second layer onlarger islands. The mean size of the islands on the terracesincreases to approximately 80–100 A and the island edgesare in general oriented along the {011} directions of thesubstrate (see Fig. 2c). In Fig. 2d–e two STM images cor-responding to 0.6 MLE Ag are shown. In this case, the for-mation of extended smooth nanowires (up to severalthousand A long) along the edges is evident. These featuresare 30–40 A wide and mostly only one layer high, but sometwo layer high sections can be identified as well. The islandson the terraces show almost the same mean size as observedin the case of the previous Ag dose (0.3 MLE), the increaseof the metal coverage leads only to an increase of the frac-tion of two layer high NPs. At the Ag coverage of 0.6 MLEthe LEED pattern of the c(4 · 2)-NiOx superstructure isstill visible, although the diffraction spots are very diffuseand the background intensity is high. On the other hand,no superstructure, originating from the NPs is detectable.

Fig. 2. Evolution of the Ag/NiOx/Pd(100) surface for increasing Ag coverages: (a) 0.15 MLE Ag, (1000 A · 1000 A, V = 0.6 V; I = 0.2 nA), the insetshows an high resolution image of a silver cluster surrounded by the c(4 · 2) structure, (75 A · 75 A, V = 1.25 V; I = 0.2 nA); (b) 0.3 MLE Ag(1000 A · 1000 A, V = 1.5 V; I = 0.1 nA); (c) 0.3 MLE Ag (250 A · 250 A, V = 1.5 V; I = 0.1 nA); (d) 0.6 MLE Ag (1000 A · 1000 A, V = 1.6 V;I = 0.1 nA); (e) 0.6 MLE Ag (250 A · 250 A, V = 0.5 V; I = 0.4 nA); (f) 1 MLE Ag (1000 A · 1000 A,V = 1.5 V; I = 0.1 nA).

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Fig. 2f shows the surface after deposition of 1 MLE Agmetal on the NiOx/Pd(1 00) substrate. The surface is cov-ered by an almost continuous ML of Ag metal scatteredon top by smaller islands and NPs. The long metal nano-wire-like features, which decorated the edges before, arenow embedded in the main Ag layer, which contains troughsalong the main crystallographic directions due to uncoveredoxide regions (see e.g. the ellipses in Fig. 2f). The LEED pat-

tern observed on this surface is a simple (1 · 1), with a highbackground and diffuse spots. No atomic resolution waspossible on the NPs or on the Ag overlayer.

3.3. XPD results

In order to obtain crystallographic information on theAg NPs and to detect any possible epitaxial ordering, we

Fig. 3. 2p XPD plots of the Ag 3d intensity (Ekin � 1119 eV) for the Ag/NiOx/Pd(100) system as a function of Ag coverage.(a) 0.3 MLE, (b) 0.8 MLE, (c)1.5 MLE. The raw intensity has been normalized with respect to 1/cosh typical of ultrathin films. The colour scale ranges from black (minimum) to yellow(maximum). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

S. Agnoli et al. / Surface Science 602 (2008) 499–505 503

have undertaken an angle-scanned XPD investigation[39,40]. In Fig. 3 we report 2p plots of the intensity ofthe Ag 3d photoelectrons (Ekin � 1119 eV) for differentcoverages. In this type of presentation, the centre of theplot corresponds to the surface normal, the radial sectiondisplays a polar scan, the circular section an azimuthal scanand the photoelectron intensity is given by the correspond-ing value of the colour scale. Exploiting the observed sym-metry, the whole set of XPD data has been fourfoldaveraged; the polar angle ranges from h = 0� to h = 66�.As the photoelectron kinetic energy is rather high, theinterpretation of the Ag 3d XPD pattern can be carriedout by a simple kinematical single-scattering model, wherethe strongest intensity enhancements are interpreted as de-rived from a forward-focusing effect [40]. Starting with thelowest coverage (Fig. 3), strong intensity maxima can beobserved at h = 45� along the [001] direction and at 34�and 55� along the [011] azimuth. This pattern remains un-changed and becomes even more evident for the highestinvestigated coverage (1.5 MLE). These findings clearlyindicate that the Ag nanostructures (either NPs or ultrathinfilm) grow azimuthally ordered, adopting the epitaxial(100)[00 1]//(10 0)[001] relationship with respect to the

Fig. 4. XPD pattern of the Ni 2p (Ekin � 634 eV) and Ag 3d (Ekin � 1119 eV)The raw intensity has been normalized with respect to 1/cosh typical of ul(maximum). (For interpretation of the references to colour in this figure legen

Pd(100) substrate. The presence of the intensity maximumat 45� along the [001] direction, which is observed alreadyfor an Ag dose of 0.3 MLE, can be explained assuming thatthe Ag NPs are at least two layers high, hence indicating a3D growth mode. Similarly, the strong intensity maximumclearly observed along the surface normal at a coverage of1.5 MLE is only compatible with the existence of a thirdlayer.

In Fig. 4 we compare the XPD patterns of the Ag 3d andNi 2p photoemission peaks for a coverage of 0.8 MLE.Both plots present the typical fingerprint of a fcc (100) ori-ented structure. However in the case of Ni, the pattern isextremely weak. If the c(4 · 2)-NiOx structure remained in-tact after the Ag deposition and provided a simple epitaxialrelationship between Ag and the underlying NiOx ML, onewould expect a strong modulation of the Ni signal, sinceAg is a strong scatterer. One has to consider that thec(4 · 2)-NiOx structure is essentially similar to aNiO(1 00) surface, hence the epitaxial relationship ofAg(100) clusters could be derived from the reverse systemNiO(1 00)/Ag(10 0) [30,32], i.e. with the metal Ag atomssitting on top of the c(4 · 2) oxygen atoms. This means thatNi and Ag atoms would follow an fcc stacking. Therefore,

peaks taken for the Ag/NiOx/Pd(100) system at a 0.8 MLE Ag coverage.trathin films. The colour scale ranges from black (minimum) to yellowd, the reader is referred to the web version of this article.)

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if the Ag atoms give rise to strong scattering events ath = 45� along the [001] direction, which is typical of atwo layer high fcc structure, in the case of Ni we would ex-pect a strong maximum along the surface normal due to athree layers high fcc structure. The absence of such a peakin the Ni pattern excludes the possibility that the c(4 · 2)-NiO phase remains unaltered after the Ag deposition. Onthe other hand, the faint XPD pattern indicates that NiOx

clusters are formed with an fcc structure characterized bysome residual order. One possible explanation is that someNi atoms are diluted in the growing Ag film similarly towhat has been reported in the case of the deposition ofAu on NiO [41]. However, this would lead to the formationof a metal-like component in the Ni 2p XPS spectrum,which is not in tune with our findings (see Fig. 1). On thecontrary, we observe a shift towards higher BE, whichwould better fit with the hypothesis of a transition of thec(4 · 2)-NiOx 2D monolayer to 3D NiO clusters, as alreadysuggested in Ref. [38].

3.4. General discussion

Both STM and XPD data clearly evidence that the Aggrowth follows a Stranski–Krastanov scheme, which is inagreement with the dewetting behaviour of Ag on bulkNiO [42]. The XPD data provide unequivocal evidencefor an epitaxial relationship of the Ag nanostructures withrespect to the Pd(100) substrate. This could be related tothe disruption of the c(4 · 2)-NiOx template as a conse-quence of the Ag deposition.

The most interesting morphological feature of the Agnanostructures is the strong tendency to decorate the uppersubstrate step edges to form long features with a nanowire-like appearance. If we look at the details of the wetting

Fig. 5. A zoom-in STM image of the step edge of the NiOx/Pd(100)surface (100 A · 100 A, V = 2.1 V; I = 0.1 nA).

behaviour of the c(4 · 2)-NiOx layer we can see that theupper rim of the step edges is often not covered by theNiOx layer: this may be seen in the STM picture ofFig. 5 where a detail of the step edge is shown [34]. If weconsider that this is the area which is decorated duringthe first stages of the Ag deposition, we can argue thatthe nanowire-like features are characterized by a directPd–Ag interaction. The presence of a Pd–Ag interfacewould also explain why at very low coverages, when mostof the Ag is concentrated at step edges, the BE of the Ag 3dis shifted to lower values (see Fig. 1) with respect to the BEtypical of bulk Ag. Actually it has been demonstrated boththeoretically and experimentally that Pd–Ag alloys andbimetallic NPs show the same behaviour [43], which origi-nates from the change in the atomic charge due to thehybridization between VB electrons [44]. On the contrary,in the case of Ag NPs on oxides, the observed BE shift isalways towards higher BE because of strain and initial stateeffects [45–47].

4. Conclusions

The growth of a metal on oxides is an extremely impor-tant topic for the development of tailored nanostructuresas well as to get some insight into heterogeneous catalysis,where metallic NPs grown on ultrathin oxide films areused as model systems. We have reported on the growthmorphology of Ag nanostructures (nanoparticles, nano-wire-like, nanolayers) deposited onto the c(4 · 2)-NiOx/Pd(100) surface. Small clusters are obtained on the ter-races, whereas the tendency of Ag to decorate the stepedges can lead to the formation of extended nanowire-likefeatures, reinforcing the general idea of using a steppedsubstrate as an efficient template for the growth of anarray of metal nanowires [48]. XPD measurements haveevidenced that the NPs adopt a fcc structure epitaxially re-lated to the substrate according to the following relation-ship: Ag(100)[001]//Pd(100)[0 01], while by means ofSTM we have found that the shape of the islands is typi-cally rectangular with the edges running along the [011]directions. The chemical situation following the Ag depo-sition resulted to be quite complex: the NiOx ML is locallydisrupted in the vicinity of the Ag NPs and gets destroyedas a function of Ag coverage and a rather disordered NiOx

phase is formed. As a consequence, the c(4 · 2)-NiOx/Pd(100) surface cannot be considered as an efficient tem-plate to stabilize metal nanoparticles against Ostwald rip-ening phenomena. On the other hand, chemicalmodifications of Ag are observed in XPS. In particular,it may be speculated that a Ag–Pd bimetallic alloy isformed which may be interesting for heterogeneouscatalysis.

Acknowledgements

This work has been funded by European Communitythrough the STRP project (Growth and Supra-organization

S. Agnoli et al. / Surface Science 602 (2008) 499–505 505

of Transition and Noble Metal Nanoclusters) and by theItalian Ministry of Instruction, University and Research(MIUR) through the fund ‘‘Programs of national rele-vance’’ (PRIN-2003, PRIN-2005). The STM experimentsin Graz are also supported by the Austrian Science Funds.

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