Generation of Double-Layer Steps on (010) Surface of Orthorhombic MoO 3 via Chemical Etching at Room Temperature Z. Y. Hsu and H. C. Zeng* Department of Chemical and EnVironmental Engineering, Faculty of Engineering, National UniVersity of Singapore, 10 Kent Ridge Crescent, Singapore 119260 ReceiVed: April 20, 2000; In Final Form: August 29, 2000 Using the methods of atomic force microscopy, X-ray diffraction and Fourier transform infrared spectroscopy, chemical etching of the (010) surface of flux-grown orthorhombic MoO 3 (R-MoO 3 ) single crystals has been investigated in detail. The (010) surface was etched repeatedly in a 0.08 M NaOH aqueous solution, and its topographies were recorded to reveal the evolution patterns upon the etching time. It is found that the “molecular steps” at a height of a single double-layer of MoO 3 (6.9 Å, or 1 / 2 b o ) can be generated on the surface of (010) in the basic solution. A “layer-by-layer” etching mechanism proposed previously has been confirmed in this model catalyst fabrication. The observed etch pit volumes and multiple-step formation have also been addressed. Although they are created at room temperature, the surface steps/edges are chemically and thermally stable in a common operating temperature range of 350-400 °C. The fabrication/modification of future catalysts via chemical etching is anticipated, since surface steps/edges are expected to be active sites in many heterogeneous catalytic reactions. Introduction In recent years, investigation on transition metal oxide and hydroxide layered materials has become an important sub-field in materials research because of many practical applications of these materials. 1-14 Molybdenum trioxide (MoO 3 ), for example, is a catalytic layered solid receiving great attention in chemical applications such as in molecular hydrogenation, oxidative dehydrogenation, epoxidation, isomerization, disproportionation, polymerization, etherification, addition, and dehydrogenation. 1-5 In addition to its prime application in heterogeneous catalysis, 6-8 MoO 3 is also widely used as an inorganic host material for preparations of photochromic and electrochromic materials and charge-density wave conductors. 9-13 And most lately, nano- structured MoO 3 has been proposed to be a new type of lubricant for load-independent friction. 14 MoO 3 can exist in two crystalline polymorphs, the thermo- dynamically stable orthorhombic R-MoO 3 and metastable monoclinic -phase (hexagonal). 15 In the R-phase, MoO 3 crystal has a layered structure with the orthorhombic symmetry (a o ) 3.963 Å, b o ) 13.86 Å, c o ) 3.696 Å) 16 and rectangular morphology, as shown in Figure 1. One of the most striking features of R-MoO 3 is that the oxygen coordination about the Mo atoms is asymmetric. The MoO 6 octahedra are considerably distorted around the central metal, with a spread of Mo-O bond distances ranging from 1.67 to 2.33 Å (Figure 1). The asym- metrical MoO 6 octahedra are interconnected through corner- linking along [100] and edge-sharing along [001] to form double-layer sheets parallel to the (010) plane (i.e., basal plane). There are only weak interactions (mostly van der Waals) between the double-layer sheets, which is reflected in the large inter-sheet distance ( 1 / 2 b o ) 6.93 Å) and the easy cleavage along the (010) plane of the crystals. 16 Concerning its catalytic performance, synthesis of R-MoO 3 materials has been an important subject for many decades. 17,18 This includes preparation of MoO 3 particles ranging from submicron scale to a few nanometers. 6,18 As a new trend, recent advances in catalytic materials take a much closer look not only at interfacial phenomena but also at the more detailed structural and crystalline features of the catalysts. In particular, the knowledge and experience of researchers in ceramic and electronic materials are having a profound impact on the fundamental understanding of the new types of catalyst process- ing. Recently, we had applied a chemical etching technique to the preparation of R-MoO 3 catalysts. 19a It was found that, upon the chemical etching, areas of {001} and {100} planes are significantly increased while surface steps, ledges, and terraces are created. Moreover, alternate crystal plane combinations of either {100} and {010}, or the {001} and {010} had been * Author to whom correspondence should be addressed. Tel: +65 874 2896. Fax: +65 779 1936. E-mail: chezhc@nus.edu.sg. Figure 1. Structure of R-MoO3 (three double layers are shown here) and orientation of crystallographic planes; Mo-O(1) ) 1.67 Å, Mo- O(2) ) 2.33 Å, Mo-O(2′) ) 1.95 Å, Mo-O(3) ) 1.73 Å, and Mo- O(3′) ) 2.25 Å. 11891 J. Phys. Chem. B 2000, 104, 11891-11898 10.1021/jp001521b CCC: $19.00 © 2000 American Chemical Society Published on Web 11/21/2000