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2008, 112, 16166–16170 Published on Web 09/25/2008
Scanning Tunneling Microscopy Study of a Vicinal Anatase TiO2 Surface Shao-Chun Li,* Olga Dulub, and Ulrike Diebold* Department of Physics, Tulane UniVersity, New Orleans, Louisiana 70118 ReceiVed: July 19, 2008; ReVised Manuscript ReceiVed: August 15, 2008
Using scanning tunneling microscopy (STM) and low-energy electron diffraction (LEED), the structure of the anatase TiO2 (51j4) surface, ∼10° vicinal to the lowest-energy (101) plane, has been studied. The surface was found to facet into a structure composed of ridges with a uniform width of five lattice units. On the basis of atomically resolved STM and electron counting rules, it is proposed that the sides of the ridges are parallel to (11j0) and (112) planes. These sides might be reconstructed to stabilize the microfaceted structure. Vapordeposited gold shows pronounced clustering between the ridges, indicating a one-dimensional template effect of the vicinal surface, which supports denser and more uniformly sized Au clusters, as compared to the flat (101) surface. Introduction Titanium dioxide finds versatile applications in various technical fields including gas sensing, coatings, pigments, heterogeneous catalysis, photocatalytic degradation of pollutants, and solar cells.1-3 This material has been studied extensively, particularly in surface science.4 TiO2 is found in three main crystallographic phases, rutile, anatase, and brookite. Rutile is the thermodynamically most stable form and is considered a model system for basic research. Surface studies on rutile have mainly focused on the (110) 1 × 1 surface, for it has the lowest formation energy. One particularly interesting aspect motivating surface studies on TiO2 is its role as a support; TiO2 promotes the catalytic activity of nanosized metal clusters such as Au.5-7 Most TiO2 nanomaterials are of anatase form, and commercial TiO2 catalysts are often a mixture of rutile and anatase. Anatase TiO2 is often considered to be catalytically more active than rutile for reasons not yet completely understood. Whereas many works have been published on the rutile system, only a few studies were done on the anatase phase due to difficulties of growing large single crystals in the laboratory.8-10 Molecular beam epitaxy (MBE), metal organic chemical vapor deposition (MOCVD), and other growth methods on appropriate substrates have been explored.11-14 Alternatively, natural mineral samples are a good choice to perform the basic research on anatase.9 The atomic structure of the lowest-energy plane, the (101) surface, has been characterized10 and shows a bulk-terminated 1 × 1 structure. The anatase (001) surface shows a (1 × 4) reconstruction,11-15 and the anatase (100) surface reconstructs to a 1 × n structure.8 Metal clustering on a flat anatase (101) was also studied recently.16 It is well-known that surface defects play a critical role in the surface chemistry of TiO2 and other metal oxides.3,4 On rutile TiO2(110), point defects consisting of isolated oxygen vacancies, which are easily formed on under-coordinated surface oxygen sites by thermally annealing in ultrahigh vacuum (UHV), can drastically influence the surface chemical properties.17-20 It was * To whom correspondence should be addressed. E-mail: (U.D.)
[email protected]; (S.-C.L.)
[email protected].
10.1021/jp806383c CCC: $40.75
observed that point defects do not form as easily on anatase (101) surfaces.10,16,21 Thus, surface steps represent some of the most common surface defects on flat anatase (101) surfaces.22 Step edge formation energies for various step edge orientations on anatase TiO2(101) have been calculated22 by analyzing the surface energy of vicinal surfaces with a systematically increased step-step separation. The computational results explain well the orientation preference of the trapezoidal terraces on the (101) surface observed experimentally.10,16 Steps, albeit with a somewhat different structure, are also particularly important for nanocrystals23 since the ratio of step atoms to volume atoms increases drastically as the crystal size approaches the nanoscale. It is thus desirable to obtain further atomic-level information about surface steps on anatase. So far, only the low-index surfaces of anatase TiO2 have been investigated,8-14,24,25 on which the terrace widths are relatively large and the interactions between step edges are negligible. Vicinal surface structures, which have a larger density of surface steps,4,26-28 have yet to be explored experimentally on anatase. The motivation for this work is two-fold, first, to broaden the knowledge of anatase surface structures by studying vicinal TiO2 surfaces and, second, to study the effect of a large step edge density on supported metal clusters. The surface step configuration was tailored by cutting a macroscopic anatase crystal ∼10° off the (101) plane, thus creating a vicinal surface with a high density of steps and, consequently, undercoordinated surface atoms. This provides an appropriate sample for investigating steps on the atomic level. The atomic structure is characterized with scanning tunneling microcopy (STM). It is shown that the vicinal surface is more complex than expected; instead of a simple bulk termination with equally spaced terraces and steps, it exhibits a microfaceted structure with pronounced ridges parallel to the [1j11] direction. The distribution of the width of these ridges is sharply peaked at ∼17.5 Å with a full width at half-maximum (fwhm) of ∼2 lattice units (∼6.3 Å) along [01j0]. It is conjectured that the side walls of the ridges are reconstructed to stabilize the microfaceted surface, although no firm experimental evidence can be obtained from STM alone. 2008 American Chemical Society
Letters
J. Phys. Chem. C, Vol. 112, No. 42, 2008 16167
Additionally, the formation of Au clusters on the vicinal surface has been studied to investigate the influence of such a ridge structure on TiO2’s role as a support.5-7 Experimental Section The experiments were carried out in an ultrahigh vacuum (UHV; base pressure
