Near-field infrared microscopy: A novel analytic mapping technique to nanocharacterize calcium silicate-based cement materials

Abstract

Infrared imaging via scattering-type scanning near-field optical microscopy (s-SNOM) allows chemical mapping of organic and inorganic materials with nanoscale spatial resolution. However, its potential adaptation to the complex multiphase structure of Portland cement is yet to be explored. Here we demonstrate the successful implementation of s-SNOM to spatially resolve coexisting chemical phases in tricalcium silicate, Portland cement's main compound, with 20-nm resolution. We found that s-SNOM is sensitive to different anhydrous polymorphic phases, revealing nanoscale domains that are ‘invisible’ to other microscopic techniques. Furthermore, s-SNOM's ability to distinguish the unhydrated and hydrated phases signifies its great promise as an analytical tool to study the complex hydration process of cement. The key to s-SNOM's application was nano-modifying the surface roughness of the cement samples, allowing nanoscale infrared imaging without topographical artifacts. Our study opens a window for infrared spectral microscopy in cement and other porous inorganic materials.

Introduction

Infrared (IR) light in the frequency range of 400–4000 cm⁻¹ (λ = 25–2.5 μm) is used to obtain vibrational information of molecular species and is an important tool for characterizing organic and inorganic materials. Far-field IR techniques such as Fourier-transform infrared spectroscopy (FTIR) have been extensively used to identify and quantify different chemical phases in ordinary Portland cement (OPC) materials (Fig. 1a) [[1], [2], [3], [4], [5], [6], [7]]. Compared with electron [[8], [9], [10]], magnetic field [11], X-ray [12], and neutron [13] characterization techniques, IR spectroscopy can provide unique information about the complex multiphase structure of OPC, including information about the molecular structure of the silicate [3], aluminate [5], and carbonated phases [7]. However, because of the diffraction limit and the wavelength of IR light, conventional far-field IR techniques have a low spatial resolution (order of several microns) [14], prohibiting direct chemical mapping of the OPC phases at the nanoscale level. Yet, advances in nanoscale analytic techniques are critical to deepening our understanding of the OPC nanostructure and enabling the development of more sustainable cements [15].

Recently, scattering-type scanning near-field optical microscopy (s-SNOM) broke through the IR light diffraction limit, opening a window for IR spectral mapping with submicron spatial resolution (also referred to as nano-FTIR). As shown in Fig. 1b, s-SNOM uses an atomic force microscope (AFM) tip as an antenna, which concentrates the incident IR into its apex [16]. When the tip's apex nears the material's surface (at a distance smaller than the apex radius), a dielectric near-field interaction between the tip and the sample modifies the amplitude s(ω) and phase φ(ω) of the elastically scattered IR light (for a complete description of near-field light–matter interactions, the readers are referred to refs. [[17], [18], [19]]). This near-field interaction is sensitive to the vibrational absorption properties of the probed material and enables spectroscopic mapping with a resolution of the size of the tip's radius (<20 nm) [14,20,21], a resolution value 2–3 orders of magnitude smaller than the wavelength of incident light and of conventional FTIR techniques.

By combining high spatial resolution with IR's sensitivity to molecular chemical properties, s-SNOM can provide nanoscale-level understanding of a wide range of materials, allowing sub-micron phase identification in geological [14], biological [22], metallic [23], polymeric [24] and semiconductive [25] samples. However, despite appearing to be a promising analytic technique to nanocharacterize the chemical phases in complex structures, the capability of s-SNOM techniques for OPC materials has not yet been demonstrated. One challenge of characterizing highly porous inorganic materials such as OPC using s-SNOM is associated with the difficulty in sample preparation. As a tip-based microscopy technique, for accurate signal detection s-SNOM requires highly flat surfaces without surface roughness artifacts [26], which are difficult to eliminate in porous materials.

Herein, we successfully implemented s-SNOM to characterize the calcium silicate phases in tricalcium silicate (C3S), the main OPC compound, with point resolution of 20 nm. To minimize topographic artifacts and enable the identification of nanoscale phases, we developed a surface nanomodification method to create C3S paste samples with a surface roughness <10 nm (down from hundreds of nanometers). After the surface nanomodification procedure, we have mapped for the first time coexisting sub-micron polymorphic phases within anhydrous C3S grains, including a rarely observed dendritic phase, revealing their structural features with spatial resolution orders of magnitude higher than other optical microscopy techniques [[27], [28], [29]]. With the ability to distinguishing the hydrated and unhydrated phases and precisely quantify the hydration degree of C3S pastes, s-SNOM can potentially enable analytical mapping of OPC's hydration and, thereby, enable a deeper understanding of the reactivity and performance implications of different cement polymorphs. Our study lays the foundation for nanoscale, analytic tip-based microscopy techniques in silicate-based cements and other porous materials and minerals.