Growth of two dimensional silica and aluminosilicate bilayers on Pd(111): from incommensurate to commensurate crystalline

Abstract

Two-dimensional (2D) silica (SiO) and aluminosilicate (AlSiO) bilayers grown on Pd(111) were fabricated and systematically studied using ultrahigh vacuum surface analysis in combination with theoretical methods, including Auger electron spectroscopy, X-ray photoelectron spectroscopy, low-energy electron diffraction (LEED), scanning tunneling microscopy (STM), and density functional theory. Based on LEED results, both SiO and AlSiO bilayers start ordering above 850 K in 2 × 10 Torr oxygen. Both bilayers show hexagonal LEED patterns with a periodicity approximately twice that of the Pd(111) surface. Importantly, the SiO bilayer forms an incommensurate crystalline structure whereas the AlSiO bilayer crystallizes in a commensurate structure. The incommensurate crystalline SiO structure on Pd(111) resulted in a moiré pattern observed with LEED and STM. Theoretical results show that straining the pure SiO bilayer to match Pd(111) would cost 0.492 eV per unit cell; this strain energy is reduced to just 0.126 eV per unit cell by replacing 25% of the Si with Al which softens the material and expands the unstrained lattice. Furthermore, the missing electron created by substituting Al for Si is supplied by Pd creating a chemical bond to the AlSiO bilayer, whereas van der Waals interactions predominate for the SiO bilayer. The results reveal how the interplay between strain, doping, and charge transfer determine the structure of metal-supported 2D silicate bilayers and how these variables may potentially be exploited to manipulate 2D materials structures.

Ultrathin silica films on Pd(111): Structure and adsorption properties

“…This conclusion is further supported by STM studies performed in another UHV chamber. At sub-monolayer coverages, silica forms irregularly-shaped islands which coalesce at increasing coverage ( Although our results generally agree with those reported by Jhang et al [16] also showing the formation of a bilayer film on Pd(111), none of our films exhibited long-range ordering. We have to recall, however, that on Ru(0001) the bilayer silica films may also form crystalline and amorphous phases (often coexisting) depending on the preparation.…”

“…[5] Such comparison would provide a deeper understanding of the role of a metal substrate on the atomic structure of a silica film and its reaction at the interface. In the course of our studies we have learned that Altman and co-workers studied the growth of silica on Pd(111) for comparison with Pd(100) [16]. The authors found that the silica bilayer starts ordering above 850 K and forms an incommensurate crystalline structure resulted in a Moire pattern.…”

Assessing the film-substrate interaction in germania films on reconstructed Au(111)

“…On high oxygen affinity metal substrates, such as Mo, silica forms only chemically bonded monolayer films [52]; on inert metals, such as Pd and Pt, the film-substrate interaction is weaker and forms amorphous decoupled (or crystalline and nonconmensurate) bilayer films [43,[53][54][55]; and on intermediate metal supports, such as Ru, exhibits both types of behaviors [56][57][58]. Jhang et al has shown by doping the silica films that the structure of the film is also heavily affected by the strain and charge transfer with the metal substrate [53]. Interestingly, silica bilayer films interact only through van der Waals forces with the metal substrate, as was determined by infrared reflection absorption spectroscopy and DFT calculations [58].…”

2D Oxide Nanomaterials to Address the Energy Transition and Catalysis

“…2D silica and silicate films can be prepared on metal substrates, including Pd(100) and Pd(111), and Ru(0001) and metal alloys . While it is good approach to study 2D materials by combination of experimental and theoretical techniques, it is not a practical method for the preparation of 2D zeolites.…”

“…Doping with Al can cause the softening of the material, reducing lattice strain energy and formation of a commensurate phase due to the stronger interaction between the metal and the aluminosilicate layer. Stronger interaction in the latter case corresponds to the electrostatic interaction and partial charge transfer between negatively charged aluminosilicate layer and the metal substrate and results in PdO bond formation . Systems prepared by deposition of silica on metal substrates do not have any silanol groups on their surface and they can be considered as perfect models for investigation of various properties using numerous surface science experimental characterizations.…”