Surface Heterogeneity in Amorphous Silica Nanoparticles Evidenced from Tapping AFM–IR Nanospectroscopy

Abstract

In this work, we propose to evaluate and validate an emerging spectroscopic space-resolved technique: atomic force microscopy coupled with infrared spectroscopy (AFM−IR) for inorganic materials in tapping mode at the nanoscale. For this aim, a preliminary investigation of sample preparation techniques was done and the stability of tapping AFM−IR spectra was evaluated on reference samples [poly(methyl methacrylate) and silica]. It was concluded that for a homogeneous polymer, it is possible to compare AFM−IR spectra with conventional Fourier-transform infrared (FTIR) spectra obtained in transmission. When an inorganic solid is considered, AFM−IR spectra are different from the global FTIR spectrum which indicates that the AFM−IR technique probes a volume which is not representative of global composition, that is, the external surface layer. Moreover, local infrared spectra recorded in the tapping mode of the external surface are significantly different depending on the analyzed regions of the same particle and between particles of the amorphous silica, implying surface heterogeneity. The AFM−IR technique allows surface description of amorphous inorganic materials at the nanoscale and opens new frontiers in the characterization of functional nanoscale and crystalline materials.

Chemical functional characterization of immature and mature coals at the nanoscale by atomic force microscopy-based infrared spectroscopy (AFM-IR)

“…In our study, the good agreement between the minerals obtained by AFM-IR tapping imaging and contact spectroscopy proves AFM-IR as a powerful and reliable tool for mineralogical characterization of coal/shale at a high spatial resolution (e.g., ~20 nm based on the 1040 cm -1 imaging of PSOC 379 coal) (Fig. S-8) (Benedis et al, 2022). It is possible to separate the phyllosilicate spectral signature from the organic matter phases (e.g., C1 and C2 spectra) in Fig.…”

“…In this respect, we have investigated the effect of two sample preparations, as pressed samples on diamond windows and sulfur embedded ultrathin sections. Measurements conditions are also a major issue, and we collected data with both contact and tapping modes (CM and TM, respectively), and discuss their consistency and respective advantages (Benedis et al, 2022). Last, and, not least, it is unclear whether AFM-IR spectra are strictly similar to µ-FTIR spectra due to the limited number of studies so far.…”

Growth dynamics and amorphous-to-crystalline phase transformation in natural nacre

“…It measures the force created in a very small interaction volume between the AFM cantilever tip and the sample surface, while a laser sweeps through the infrared spectrum causing absorption-specific variations in the recorded force 42 . PiFM absorption spectra agree with those produced by traditional FTIR spectroscopy 43 but are obtained at a higher spectral and spatial resolution, which leads to some differences in absorption band appearence 44 particularly for materials that are heterogeneous at the nanoscale.…”