Plasma Functionalization of Silica Bilayer Polymorphs

Abstract

Ultrathin silica films are considered suitable two-dimensional model systems for the study of fundamental chemical and physical properties of all-silica zeolites and their derivatives, as well as novel supports for the stabilization of single atoms. In the present work, we report the creation of a new model catalytic support based on the surface functionalization of different silica bilayer (BL) polymorphs with well-defined atomic structures. The functionalization is carried out by means of in situ H-plasma treatments at room temperature. Low energy electron diffraction and microscopy data indicate that the atomic structure of the films remains unchanged upon treatment. Comparing the experimental results (photoemission and infrared absorption spectra) with density functional theory simulations shows that H₂ is added via the heterolytic dissociation of an interlayer Si-O-Si siloxane bond and the subsequent formation of a hydroxyl and a hydride group in the top and bottom layers of the silica film, respectively. Functionalization of the silica films constitutes the first step into the development of a new type of model system of single-atom catalysts where metal atoms with different affinities for the functional groups can be anchored in the SiO₂ matrix in well-established positions. In this way, synergistic and confinement effects between the active centers can be studied in a controlled manner.

Keywords: crystalline; hydride; hydroxyl; plasma functionalization; ultrathin silica film; vitreous.

Figure 1

(a,b) LEEM images and (c,d) LEED patterns of the SiO₂ BL/Ru(0001) samples in the (a,c) pristine state and after (b,d) 64 min of H-plasma exposure. Electron energy: 42 eV.

Figure 2

(a) O 1s and (b,c) Si 2p XPS lines for a crystalline SiO₂ BL/Ru(0001) sample acquired after consecutive and incremental H-plasma exposures, as indicated. O 1s and Si 2p lines were collected with photon energies of 600 and 175 eV, respectively. (c) Superposition of Si 2p lines for the pristine SiO₂ BL and 64 min plasma-treated sample. (d) Time evolution of the components’ areas obtained from the sample fitting of the Si 2p lines for the different silica samples. Fitting results of the complete time series are available in Section 4 of the Supporting Information.

Figure 3

IRAS spectra of a crystalline SiO₂ BL/Ru(0001) showing the (a) O–D, (b) Si–D, and (c) Si–O–Si vibration modes under different H-plasma treatment stages. The operating anode voltages were 400 V (mild) and 800 V (strong). Consecutive plasma exposure time was 20 min in both cases. The plasma source was operated with deuterium at a pressure of 1.2 × 10^–4 mbar. The dashed lines indicate the positions of the absorption lines.

Figure 4

Top panel: atomic model of the functionalized silica BL showing the position of −D and −OD groups. Bottom panel: IRAS spectra calculated for the structures shown in the top panel. Semitransparent blue boxes indicate the regions where the vibration signals were experimentally observed.