8024
J. Phys. Chem. C 2007, 111, 8024-8029
Ordered Arrays of Au Nanoclusters by TiOx Ultrathin Templates on Pt(111) Francesco Sedona,† Stefano Agnoli,† Mattia Fanetti,‡ Iskandar Kholmanov,‡,§ Emanuele Cavaliere,‡ Luca Gavioli,‡,§ and Gaetano Granozzi*,† Dipartimento di Scienze Chimiche and Unita` di Ricerca CNR-INFM, UniVersita` di PadoVa, Via Marzolo, I-35131 PadoVa, Italy, Dipartimento di Matematica e Fisica, UniVersita` Cattolica, Via dei Musei 41, I-25121 Brescia, Italy, and Laboratorio Nazionale CNR-INFM-TASC, S.S. 14 Km 163.5, 34012 BasoVizza (TS), Italy ReceiVed: December 20, 2006; In Final Form: March 26, 2007
Ultrathin ordered films of TiOx (x ≈ 1) on Pt(111) have been investigated by scanning tunneling microscopy (STM) to test their capability as templates for growing ordered and monodispersed Au nanocluster arrays. The ordered array of parallel black troughs spaced by 1.44 nm, observed in the STM data of the zigzag-like TiOx ultrathin film, was revealed to be a good template for growing a linear array of Au clusters, with a mean size of 1.3 nm and a narrow dispersion. The wagon-wheel-like TiOx film, having a similar chemical composition but without a nanostructured array of defects, does not show templating effects, thus leading to nucleation of disordered and larger Au clusters (mean size of 3.4 nm with a large dispersion). Hence, this work shows that the ordering of the deposited Au nanoclusters is strongly dependent on the actual defectivity of the film. Annealing the Au cluster arrays at high temperature produces drastic changes in the spatial arrangement of the clusters, which has been interpreted as the consequence of the changes in the ultrathin film template.
1. Introduction A main issue of modern nanoscience is to obtain ordered arrays of well-defined and uniformly sized (almost monodispersed) nanostructures, e.g., nanodots and nanowires. In addition to the enormous interest toward the downsizing process itself, new and potentially innovative optical,1 magnetic,2 and catalytic properties3 of these nanostructures is also gaining attention. The nanofabrication, i.e., the control of the shape, dimensions, and order of such nanostructures, can be obtained with the wellknown sequential top-down approach, e.g., by nanolithographic methods or by atomic manipulation via scanning tunneling microscopy (STM).4 Alternatively, size-selected cluster deposition methods could be used, even if cluster ordering is difficult to achieve.5, However, the current frontier is associated with obtaining such nanostructures by a parallel self-assembly fast process. One of the most simple and promising approaches to obtain long-range self-assembled structures is related to the use of ordered arrays of defects, which act as templates for the preferential nucleation phenomenon. Such a general approach has been investigated largely when dealing with reconstructed single-crystal metal surfaces,6 and a number of ordered defects, such as steps, misfit dislocations, different stacking, and vacancies, have been found to be active as templates for metalbased nanostructures. However, metal-based templates are somewhat far from the real world because in most cases the template itself is stable only in ultrahigh-vacuum (UHV) conditions. From this point of view, oxide-based templates are more promising because in many cases they are stable systems which survive at ambient * To whom correspondence should be
[email protected]. † Universita ` di Padova. ‡ Universita ` Cattolica. § Laboratorio Nazionale CNR-INFM-TASC.
addressed.
E-mail:
conditions. Metal-cluster growth on oxide surfaces is currently a very active field of research,7,8 and examples have been reported where oxide-based ultrathin films act as templates for cluster9-12 or even single-metal atom growth.13 Within such a topic, Au nanoclusters supported on reducible metal oxides represent a subtopic of particular relevance because they are very active catalysts for a variety of reactions. Many efforts have been undertaken in order to shed some light upon the effect of cluster size and the role of the support on the catalytic activity. To this aim, surface scientists are undertaking a two-fold effort: on one side, by developing model systems as near as possible to a real working catalyst and, on the other, by synthesizing a new catalyst tailored ad hoc for a desired reaction.14 A paradigmatic example in this sense is the new, superior catalytic activity toward CO oxidation at room temperature (RT) discovered for an Au bilayer grown on reduced titania surfaces, the explanation of which, however, is still the object of debate.15-19 To provide a real step forward on this problem, we think that a detailed investigation on well-defined model systems is needed. Recently, we have been much involved in the preparation and characterization of nanostructured ultrathin films of TiOx on Pt(111).20 By a careful choice of the preparation conditions we have been able to obtain several different phases having rather different topography and degree of long-range ordered defectivity, which potentially might be used as templates for metal-cluster growth. In particular, it was shown that, by varying the growth parameters, different TiOx ultrathin films about 1 monolayer (ML) thick can be obtained, each having a distinct low-energy electron diffraction (LEED) pattern. Atomically resolved STM images have shown that they have a rather different and peculiar habitus, e.g. either a wagon-wheel-like, a kagome´ -like, or a zigzag-like contrast.20,21 In the present contribution we report a study where a welldefined TiOx phase, having a zigzag-like structure, has been
10.1021/jp0687652 CCC: $37.00 © 2007 American Chemical Society Published on Web 05/12/2007
Ultrathin Films for Growing Au Nanocluster Arrays
J. Phys. Chem. C, Vol. 111, No. 22, 2007 8025
tested as a possible template to prepare an ordered array of Au nanoclusters. According to photoemission evidence, this phase has a stoichiometry close to that of TiO.22 It is here demonstrated that the peculiar topographic features of the investigated TiOx phase are really effective in obtaining the growth of a spatially ordered array of Au nanoclusters showing a narrow size dispersion. The temperature evolution of the ordered array has been investigated, and some preliminary results indicate that the nanocluster pattern changes can be interpreted as a consequence of the structural transformation of the original template. 2. Experimental Section The preparation and the structural characterization for the different TiOx films on Pt(111) have been reported in detail elsewhere.20 In the following we will concentrate on a particular phase which, according to the notation therein reported, we call as z′-TiOx. This phase can be obtained with the following procedure (see eq 1): (1) an intermediate film is prepared at RT by Ti reactive deposition in the presence of oxygen (pO2 ) 10-4 Pa), with an initial Ti coverage of about 0.8 MLE (1 MLE corresponds to 1.5 × 1015 atoms cm-2, as determined with a quartz microbalance); (2) the intermediate film is subsequently annealed for 10 min in UHV at 673 K. RT, pO < 10-4 Pa 2
Pt(111) + 0.8 MLE Ti 98 673 K, pO < 10-8 Pa 2
intermediate film 98 z′-TiOx/Pt(111) (1) The predominant presence of such a z′-TiOx/Pt(111) phase in the reported experimental conditions is established by the existence of its peculiar LEED pattern.20 Typically, in different experimental conditions the simultaneous presence of different phases is obtained, as demonstrated by the observation of superimposed LEED patterns. Au clusters have been evaporated from a filament basket on the substrate held at RT under UHV conditions, with a typical deposition rate of about 0.3 ML/min. Actual coverages reported in the following were determined by analyzing the STM images. The sample preparation and characterization were performed using an Omicron STM/SEM/SAM UHV system with a base pressure
