Paper SS+AS-TuA9
Tungsten Trioxide Monolayer on Pd(100)

Tuesday, November 8, 2016, 5:00 pm, Room 104E

Tungsten trioxide (WO₃) is a key material in several applications including smart windows technology, photo-electrochemical water splitting, gas sensors and heterogeneous catalysis. In particular, tungsten oxides are important acid-base and redox catalysts, and they show excellent activity for many catalytic reactions, such as alcohol dehydrogenation, alkane hydrogenation and metathesis [1]. WO₃ has been produced in single crystal form or as supported thin films with the bulk crystal structure. Recently, the formation of an ordered two-dimensional (2D) tungsten oxide layer on Pt(111) has been reported, where W atoms show a mixture of 5+ and 6+ oxidation states [2].

Here we report on the preparation of a well-ordered 2D WO₃ layer on a Pd(100) surface and the characterization of its geometric, electronic and vibrational structure by a combination of STM, LEED, XPS, HREELS measurements, supported by DFT calculations. The WO₃ monolayer on Pd(100) surface and features a surface network consisting of small (~ 4 nm) square-shaped domains, separated by narrow (~ 0.3 nm) trenches (Fig. 1a). The latter are identified as anti-phase domain boundaries, as evidenced by atomically-resolved STM images (see inset of Fig. 1a) and the characteristic spot splitting in the LEED pattern (Fig. 1b). The STM image shows that each domain exhibits a square surface structure with a lattice constant of 0.39 nm, which corresponds to a c(2x2) superstructure. Another important feature is the presence of few dark depressions inside the domains, which we attribute to missing terminal O atoms (see model in Fig. 1d), in corroboration with HREELS results and high-resolution W 4f core-level spectra (Fig. 1c). The latter consist of three 4f_7/2 - 4f_5/2 doublet components, due to W atoms at different surface locations: within the defect-free areas (major component at 34.4 eV), with missing terminal oxygens (minor component at 33.3 eV), and at the domain boundaries (35.2 eV). The DFT derived structure model of the WO₃ monolayer is shown in Fig. 1d and consists of a layer of O atoms adsorbed in on-top Pd positions, followed by a c(2x2) layer of W atoms, which are connected at the top to terminal O atoms via strong W=O bonds, as suggested by the HREELS results. It can be viewed in a way as a 2D analogue of a cubic WO₃(001) crystal, featuring a similar lattice constant (0.39 nm vs. 0.38 nm) and polyhedral linkage, but with a modified W-O coordination sphere due to the contact with the Pd(100) surface.

[1] D. Gazzoli et al, J. Phys. Chem. B 101 (1997) 11129

[2] Z. Li, et al, J. Phys. Chem. C 115 (2011) 5773

This work has been supported by the FWF Project P26633-N20 and by the EU COST Action CM1104.