J. Phys. Chem. C 2010, 114, 17069–17079
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Growth and Characterization of Two-Dimensional FeO Nanoislands Supported on Pt(111)† Yunxi Yao, Qiang Fu,* Zhen Wang, Dali Tan, and Xinhe Bao* State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, The Chinese Academy of Sciences, Dalian 116023, People’s Republic of China ReceiVed: April 23, 2010; ReVised Manuscript ReceiVed: July 13, 2010
Two-dimensional (2D) FeO nanoislands with a well-controlled size, density, and surface structure have been grown on Pt(111) by a two-step preparation process, which consists of Fe deposition at low temperatures, such as 150 K, in an O2 atmosphere and subsequent annealing at elevated temperatures in ultrahigh vacuum. The atomic structure, chemical composition, and electronic state of the formed FeO nanoislands were investigated by scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy, and high-resolution electron energy loss spectroscopy. The formation of the metastable 2D FeO surface phase can be attributed to confinement effects at interfaces between nanostructured oxides and metal substrates, which originate from the strong interaction between FeO and Pt(111). Furthermore, the STM and scanning tunneling spectroscopic data indicate that the formed Pt-FeO boundaries or edges of the FeO nanoislands present distinct chemical and electronic characteristics, which could be highly active in many catalytic processes. 1. Introduction Metal oxides are widely used as catalysts or catalyst supports in many industrial reaction processes. Understanding the role of oxides in catalytic reactions is one of the most important tasks in heterogeneous catalysis research. Nanostructured oxides can be grown on metal substrates and form ultrathin films, nanoislands, or nanoclusters. Metal-supported oxide nanostructures present ideal model surfaces to study the structure-reactivity relationships in oxide catalysis.1-5 Ultrathin oxide films with a thickness of a monolayer (ML) or a few monolayers grown on conducting supports could circumvent the charging effect encountered in bulk oxides. The surface structure and thickness of these oxide films can be controlled at the atomic scale. They serve as ideal supports for metal deposition and allow investigations of the model catalysts using most of the surface techniques.6-9 Well-ordered oxide films, such as Al2O3/NiAl(110),10 FeO/Pt(111),2,11-15 MgO/Mo(100),16,17 and SiO2/Pd(100),18 have been extensively studied in past decades. Additionally, planar oxide nanoislands or nanoclusters with coverage of a submonolayer can be grown on metal surfaces to form the so-called inverse model systems.4 Oxide nanoislands, such as vanadium oxides,4,19 ceria,20,21 MoO3,22,23 and iron oxides,1,24-26 have been deposited on noble metal surfaces. Coordinatively unsaturated metal sites at edges of the nanoislands or nanoclusters are often regarded as active sites in catalytic oxidation reactions.1,22,27 In the metal-supported two-dimensional (2D) oxide nanostructures, the formed metal-oxide interfaces play an important role in the surface physical and chemical properties. For example, the interfaces between the ultrathin oxide films and the metal supports could affect the reactivity of metal catalysts grown on the oxide surfaces.3 In the systems of metal-supported oxide nanoislands, the peripheries of oxide nanoislands, that is, the metal-oxide boundaries, are often controlling catalytic reactions.1,20,28 To study the critical role of the metal-oxide interfaces on the surface reactions, †
Part of the “D. Wayne Goodman Festschrift”. * To whom correspondence should be addressed. Tel: +86-41184686637. Fax: +86-411-84694447. E-mail:
[email protected] (Q.F.),
[email protected] (X.B.).
it is highly demanding to prepare the 2D oxide nanostructures with the desired structure, morphology, and size. In the past decades, many attempts have been made to achieve the well-controlled growth of 2D oxide nanoparticles and clusters on the surfaces of metals or oxides. For example, planar vanadium oxide clusters were formed on Rh(111) by deposition of V at the submonolayer under specific substrate temperatures and oxidizing conditions.29 2D MoO3 nanoislands were deposited on Au(111) by vapor deposition of Mo and subsequent oxidation in an oxidant atmosphere.22,23 Recently, FeO was grown on Au(111) by exposing Fe overlayers to O2 at room temperature (RT), followed by annealing at elevated temperatures, and the reconstructed Au(111) surface serves as an ideal template to grow the highly dispersed 2D FeO nanoislands.24 The widely used routes to preparing the supported nanostructured oxides, including oxidation of alloy surfaces, evaporation of metals under oxidant atmospheres at evaporation of metals followed by postoxidation,8,14,30,31 often need high temperatures, which cause less controlling of the oxide growth. In this paper, we present a two-step growth method to prepare nanostructured FeO on Pt(111), which consists of deposition of Fe in an O2 atmosphere at low temperatures (LTs) and subsequent annealing at intermediate temperatures ( hcp > fcc at the bias voltage of 2.0 V. Figure 16c,d shows STM images of two FeO nanoislands recorded at low bias voltages (20-50 mV). The island in Figure 16c has the same geometry as the islands 2-8 in Figure 16a. According to the assignment for the above high-bias STM
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Yao et al. STM images and the STS results (Figure 14), the island edges show a distinct atomic structure and electronic property, which may provide active sites for catalytic reactions, for example, CO oxidation reactions.1 4. Conclusions
Figure 17. XPS Fe 2p spectra and the STM images from iron oxide overlayers on Pt(111) by depositing 0.25 ML Fe on HOPG (a) and Pt(111) (b) surfaces at 170 K in 5.0 × 10-7 mbar O2 and subsequently annealing in UHV up to 300 and 473 K, respectively. (a) 50 nm × 50 nm, 3.5 V, 0.02 nA; (b) 40 nm × 40 nm, -0.2 V, 0.03 nA.
images, the well-resolved domain in the center of the triangleshaped FeO nanoisland should be the top domain, and the adjacent three domains inside of the island and with highest brightness are fcc domains. The hcp domains are close to the edges. On the basis of our assignment, the brightness of the domains in the low-bias STM images increases in the order of fcc > hcp, which is consistent with Giordano’s results39 but contrary to the recent DFT calculations.47 The characterization results of the FeO nanoislands suggest that, in the case of submonolayer coverage, FeO nanoislands always have a monolayer thickness (actually a bilayer of Fe and O atoms) with one layer of Fe atoms at the interface and one layer of O atoms at the top surface (Figures 11-13). To illustrate further the critical role of the FeO-support interaction in the formation of the metastable FeO surface phase, iron oxides were grown on a highly orientated pyrolytic graphite (HOPG) surface using the same process as the growth of the FeO nanoislands on Pt(111). The Fe 2p3/2 BE of the FeO1+x/HOPG surface has a +2.0 eV shift in comparison with that of the FeO/ Pt(111) surface. 3D FeO1+x (x e 0.5) nanoparticles were obtained on the HOPG surface, as confirmed by the STM and XPS results (Figure 17). Obviously, the weak interaction between Fe and HOPG could not stabilize the metastable 2D FeO phase. Monolayer FeO overlayers have been grown on Pt(111),2 Ru(0001),31 and Au(111),24 and we believe that the strong interaction of Fe with these metal supports contributes to the growth of the metastable FeO surface phase. The existence of transition metal thin films is attributable to the extra stability that arises from its adhesion energy at the underlying oxidemetal interface.48 We have studied the interfacial interaction between the FeO overlayer and the Pt(111) substrate using DFT calculations, and the calculated interfacial adhesion energy is 1.40 eV per FeO formula.1 Thus, the interface confinement effect, which originates from the strong interaction between the FeO overlayer and the Pt(111) substrate, stabilizes the FeO nanoislands. The existence of a strong interface interaction is also consistent with previously published theoretical calculation results, which report that there is an electron transfer of 0.2-0.3 e from the FeO film toward the Pt(111) surface.39 Finally, it should be noted that, in comparison to the full FeO monolayer films, the main structural characteristic of the FeO nanoislands is the presence of high-density metal-oxide boundaries or island edge sites. As shown by the high-resolution
A two-step deposition process, which consists of evaporation of Fe in an oxidant atmosphere at low temperatures and subsequent annealing in UHV at elevated temperatures, was applied to grow FeO nanostructures on Pt(111). Well-defined and highly dispersed FeO nanoislands have been obtained by depositing 0.25 ML Fe at the Pt(111) substrate temperature of 150-250 K and in the O2 partial pressure range from 1.3 × 10-8 to 1.3 × 10-6 mbar, followed by postannealing between 473 and 673 K in UHV. It has been shown that the low deposition temperatures and higher oxidant atmospheres are critical to the structure and morphology of the FeO nanoislands, which facilitate the feasible oxidation of deposited Fe and the high dispersion of the formed FeO nanoislands. The well-defined FeO nanoislands exhibit a Moire´ pattern with a periodicity of 2.48 nm. The brightness order of the three stacking domains within one Moire´ pattern is top > hcp > fcc at high imaging potentials (>1.5 V). The top domains are the first ones that are not fully developed at the FeO island boundaries, whereas the fcc and hcp domains are populated inside of the islands, indicating that the thermodynamic stability of the domains increases in the order of fcc > hcp > top. At the FeO-Pt interfaces, there exist a distinct atomic structure and electronic states in comparison to the sites inside the islands. The formation of a metastable 2D FeO surface phase is attributed to the interface confinement effect, which originates from the strong interaction between FeO and the Pt substrate. The 2D interface confinement at metal surfaces results in the formation of various metastable oxide surface phases on metal substrates. Acknowledgment. This work was financially supported by the National Natural Science Foundation of China (Nos. 20733008 and 20873143), the Ministry of Science and Technology of China, and the Chinese Academy of Sciences (“Bairen” program). We acknowledge the fruitful discussions with Weixue Li and Ding Ma and the help from Weihua Wang, Hui Zhang, Bing Wang, and Jianguo Hou of the University of Science and Technology of China for the LT-STM measurements. References and Notes (1) Fu, Q.; Li, W. X.; Yao, Y. X.; Liu, H. Y.; Su, H. Y.; Ma, D.; Gu, X. K.; Chen, L. M.; Wang, Z.; Zhang, H.; Wang, B.; Bao, X. H. Science 2010, 328, 1141. (2) Weiss, W.; Ranke, W. Prog. Surf. Sci. 2000, 70, 1. (3) Freund, H. J. Surf. Sci. 2007, 601, 1438. (4) Schoiswohla, J.; Socka, M.; Chenb, Q.; Thorntonb, G.; Kressec, G.; Ramseya, M. G.; Surneva, S.; Netzer, F. P. Top. Catal. 2007, 46, 137. (5) Freund, H. J.; Goodman, D. W. Handb. Heterog. Catal. 2008, 3, 1375. (6) Freund, H. J. Surf. Sci. 2002, 500, 271. (7) Campbell, C. T. Surf. Sci. Rep. 1997, 27, 1. (8) Fu, Q.; Wagner, T. Surf. Sci. Rep. 2007, 62, 431. (9) Chen, M. S.; Goodman, D. W. Science 2004, 306, 252. (10) Ba¨umer, M.; Freund, H. J. Prog. Surf. Sci. 1999, 61, 127. (11) Galloway, H. C.; Benı´tez, J. J.; Salmeron, M. Surf. Sci. 1993, 298, 127. (12) Schedel-Niedfig, T.; Weiss, W.; Schlo¨gl, R. Phys. ReV. B 1995, 52, 17449. (13) Galloway, H. C.; Sautet, P.; Salmeron, M. Phys. ReV. B 1996, 54, R11145. (14) Ritter, M.; Ranke, W.; Weiss, W. Phys. ReV. B 1998, 57, 7240. (15) Ma, T.; Fu, Q.; Su, H.; Liu, H.; Cui, Y.; Wang, Z.; Mu, R.; Li, W.; Bao, X. ChemPhysChem 2009, 10, 1013.
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