Structural and Mechanical Properties of Silica Mesoporous Films Synthesized Using Deep X-Rays: Implications in the Construction of Devices

Paula Y Steinberg,Diego F Lionello,Benedetta Marmiroli,Heinz Amenitsch,M Mercedes Zalduendo,Leticia P Granja,Daiana E Medone Acosta,M Cecilia Fuertes,Paula C Angelomé

doi:10.3389/fmats.2021.628245

Abstract

In recent years, the use of X-Rays (XR) irradiation for the production of ordered mesoporous thin films has been well established. This technique allows obtaining porous materials that contain thermal sensitive moieties or nanoparticles. Additionally, in combination with lithographic masks, the generation of high aspect ratio patterns of several geometrical shapes with micrometric resolution is possible. In this work, the structural and mechanical properties of porous silica thin films obtained by sol-gel method along with the exposure to high intensity XR is presented. Two templates (CTAB and Brij 58) and several irradiation doses and post-synthesis treatments were evaluated by a combination of characterization techniques, including grazing incidence small-angle XR scattering, electronic microscopies, XR reflectometry and nanoindentation. The results demonstrate that all the irradiated oxides presented a highly ordered mesoporous structure, independently of the XR dose and post thermal treatment. Their mechanical properties, on the other hand, clearly depend on the irradiation dose; high hardness values were measured on samples irradiated at low doses but higher doses are necessary to obtain films with indentation modulus values similar to the obtained for thermally treated coatings. The accessible porosity, essential for the application of these films in devices for micro- and nanofluidics, is also dependent on the dose and the thermal treatment performed afterward. The same tendency is observed for the films contraction and rigidity. After this characterization, it was concluded that thermal treatments are needed after the consolidation with XR to increase the accessibility and structural integrity of these porous oxides. Finally, the production of composites with metallic (Au and Ag) nanoparticles was tested which envisioned their applications in sensing and catalysis. Moreover, diverse geometrical patterns of both pure and Ag nanoparticles doped silica mesostructured films were obtained, demonstrating the feasibility of the proposed approach. The results presented in this work are of great importance to understand the transport mechanisms that operate in these silica porous films, in order to integrate them in different devices for lab-on-a-chip applications.

Highlights

Mesoporous oxides, and silica ones, have been widely studied in the last years for transport related applications mainly due to their high surface-volume ratio and their ordered porous structure, which allows the rapid diffusion of analytes
An improved strategy, combining XR irradiation with postthermal treatments, to obtain well-consolidated, robust and accessible mesoporous silica films templated with CTAB or Brij 58 and devices derived from them, was presented in this work
Transmission electron microscopy (TEM) and Grazing-incidence small-angle XR scattering (GI-SAXS) measurements revealed that the pore ordering of these systems is not affected by the XR dose used

Summary

Introduction

Mesoporous oxides, and silica ones, have been widely studied in the last years for transport related applications mainly due to their high surface-volume ratio and their ordered porous structure, which allows the rapid diffusion of analytes. Two procedures can be used to consolidate the inorganic walls and remove the organic template to originate the porous structure: 1) thermal treatments at moderate temperature (300–500°C) (Grosso et al, 2000; Soler-Illia et al, 2012; Lionello et al, 2017); 2) a low temperature (130–200°C) thermal treatment followed by template elimination using chemical dissolution in a proper solvent (Escobar et al, 2017; Gonzalez Solveyra et al, 2017; Giménez et al, 2020) In these two cases, the contraction of the films in the direction perpendicular to the substrate promotes the interconnection of the pores, which generates the accessible mesoporosity; this feature is usually required for the integration of these films in functional devices. An increase in porosity leads to a reduction in elastic modulus and hardness; this issue represents a challenge to be addressed in the field of thin films nanomechanics (Dubois et al, 2008; Zhou et al, 2011; Vanstreels et al, 2012)

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