Mesoscopic Structures and Coexisting Phases in Silica Films

Kristen M Burson,Daniel S Wall,David Kuhness,Zechao Yang,Matthias Brinker,Wolf-Dieter Schneider,Thomas Marsh,Leonard Gura,Hans-Joachim Freund,Hyun Jin Yang,Markus Heyde

doi:10.1021/acs.jpcc.1c10216

Kristen M Burson, Daniel S Wall + Show 9 more

Open Access

https://doi.org/10.1021/acs.jpcc.1c10216

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Abstract

Silica films represent a unique two-dimensional film system, exhibiting both crystalline and vitreous forms. While much scientific work has focused on the atomic-scale features of this film system, mesoscale structures can play an important role for understanding confined space reactions and other applications of silica films. Here, we report on mesoscale structures in silica films grown under ultrahigh vacuum and examined with scanning tunneling microscopy (STM). Silica films can exhibit coexisting phases of monolayer, zigzag, and bilayer structures. Both holes in the film structure and atomic-scale substrate steps are observed to influence these coexisting phases. In particular, film regions bordering holes in silica bilayer films exhibit vitreous character, even in regions where the majority film structure is crystalline. At high coverages mixed zigzag and bilayer phases are observed at step edges, while at lower coverages silica phases with lower silicon densities are observed more prevalently near step edges. The STM images reveal that silica films exhibit rich structural diversity at the mesoscale.

Highlights

As is well-known in zeolite catalysis,[1,2] performing reactions in confined spaces presents an exciting route to influencing reaction rates and selectivity
Substrate-supported two-dimensional (2D) materials have recently garnered the interest of the scientific community as ideal model systems for studying confined space reactions due to their well-defined spatial confinement parameters and relative structural simplicity which enable comparison with theoretical models.[3−12]
Because the reaction rates can be influenced by the degree of confinement, rates of intercalation, and the presence of structural defects in thin films,[14−18] interpreting the results of confined space reaction studies necessitates a detailed understanding of the confinement structures themselves at multiple length scales

Summary

Introduction

As is well-known in zeolite catalysis,[1,2] performing reactions in confined spaces presents an exciting route to influencing reaction rates and selectivity. Theoretical and experimental studies have explored the impact of such spatial confinement on reaction rates, activation energies, and adsorption energies for common molecular reactions such as CO oxidation, water formation, and the hydrogen evolution reaction. The activation energy of CO oxidation is reduced for both graphene/Pt(111)[3] and BN/ Pt(111)[4] compared to bare Pt(111). Water formation from H2 and adsorbed oxygen on Ru(0001) exhibits a lower apparent activation energy[5] and an accelerated rate[6] when confined beneath a two-dimensional vitreous silica bilayer, BL-silica/. Because the reaction rates can be influenced by the degree of confinement, rates of intercalation, and the presence of structural defects in thin films,[14−18] interpreting the results of confined space reaction studies necessitates a detailed understanding of the confinement structures themselves at multiple length scales

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