Nanointerfaces: Concepts and Strategies for Optical and X-ray Spectroscopic Characterization

Abstract

Interfaces at the nanoscale, also called nanointerfaces, play a fundamental role in physics and chemistry. Probing the chemical and electronic environment at nanointerfaces is essential in order to elucidate chemical processes relevant for applications in a variety of fields. Many spectroscopic techniques have been applied for this purpose, although some approaches are more appropriate than others depending on the type of the nanointerface and the physical properties of the different phases. In this Perspective, we introduce the major concepts to be considered when characterizing nanointerfaces. In particular, the interplay between the characteristic length of the nanointerfaces, and the probing and information depths of different spectroscopy techniques is discussed. Differences between nano- and bulk interfaces are explained and illustrated with chosen examples from optical and X-ray spectroscopies, focusing on solid-liquid nanointerfaces. We hope that this Perspective will help to prepare spectroscopic characterization of nanointerfaces and stimulate interest in the development of new spectroscopic techniques adapted to the nanointerfaces.

Figure 1

Schematic representation of the four classes of nanointerfaces and bulk interface between a solid (A) and a liquid (B) phase. A characteristic length in the nanoscale range is highlighted in red for all interfaces. A magnified view of the bulk interface is shown to highlight the different scaling factor compared to nanointerfaces. The volume affected by interfacial phenomena, corresponding to the interfacial region, or interphase, is presented in a different color. (b) Classification of nanointerfaces based on the characteristic lengths (nano or non-nano) of the phases A and B.

Figure 2

Historical advancement of the “classical” EDL theory, showing the theories developed by Helmholtz (a), Guoy and Chapman (b), and Stern (c). Reproduced with permission from ref (17). Copyright 2009 Royal Society of Chemistry.

Figure 3

Comparison of the double layer in the “classical“ sense (a) and in a nanoconfined space (b). Reproduced with permission from ref (21). Copyright 2010 Royal Society.

Figure 4

Schematic representation of probing (grey) and information (green) depths and volumes on flat (a) and nanostructured (b) surfaces. An example is given for a photon-in electron-out spectroscopy technique such as XPS. The characteristic length of the nanostructures is of the same order of the attenuation length (AL) of the incident photons in this material.

Figure 5

(a) IR absorbance water confined in CNTs at different relative humidities. A significant contribution from loosely bonded water was observed both in small diameter nanotubes, arising from a one-dimensional chain structure, and larger diameter nanotubes, arising from a water layer next to the nanotube wall. Adapted from ref (62). Copyright 2016 American Chemical Society. (b) Water confined in imogolite nanotubes with different surface chemistry. Water confinement was found to be governed by the hydrophilicity of the inner walls, and the interaction between water and the nanotube wall affected the degree of H-bonding between water molecules. Adapted with permission from ref (63). Copyright 2018 Springer Nature.

Figure 6

DRIFTS investigation of a MOF for heat-exchange applications. 1-dimensional water chains were identified at low pressure and bulk-like water filling the pores at high pressure. (A) Illustrations of the crystal structure of the MOF, the Cr³⁺ trimer, and the organic ligand. (B) DRIFTS spectra of the MOF at different air pressures. (C) DRIFTS spectrum of H₂O···Cr and (H₂O)n···H₂O···Cr in the MIL-101(Cr) at 6.0 × 10² Pa (P_air). MD simulated vibration spectra for (D) the 41 water molecules (lower figure, each color line represents one water molecule) as well as the sum of all of the spectra (upper figure, in green) and (E) the 216 water molecules (lower figure, each color line represents one water molecule) as well as the sum of all of the spectra (upper figure, in blue). MD simulated vibration spectra of (F) single water molecule coordinated with the saturated (dark color) and unsaturated Cr³⁺ sites (light color), and (G) the first water molecule in the water chains for the 29 Å (dark color) and 34 Å cages (light color). Adapted from ref (77). Copyright 2021 American Chemical Society.

Figure 7

Nanoconfined water in bubbles between diamond and graphene probed by ATR-FTIR. (a) Schematic representation showing water cluster in graphene nanobubble (GNB) and weakly interacting water molecules underneath flat graphene on diamond (top panel). Etching of diamond by supercritical water (bottom panel). FTIR spectra showing OH-stretching peak of water measured on (b) diamond, where raising temperature to 373 K results in the desorption of water, (c) flat graphene on diamond showing peak at 3650 cm^–1 due to the presence of trapped, weakly bonded water molecules, and (d) (i) flat graphene on diamond, (ii–vi) sample after formation of GNBs on diamond and heating the GNB at a range of temperatures, and (vii) sampe after cooling down to room temperature. Reprinted with permission from ref (83). Copyright 2013 Springer Nature.

Figure 8

Nanoconfined water and protons in the nanoslits of layered Ti₃C₂ MXene. (A–C) Operando FTIR measurements of intercalated protons during electrochemical cycling in dilute sulfuric acid electrolyte. (D–F) Operando FTIR measurements of intercalated water during electrochemical cycling in highly concentrated LiCl electrolyte. Panels a–c adapted from ref (87). Copyright 2023 Springer Nature CC-BY 40. Panels d–f adapted from ref (88). Copyright 2023 American Chemical Society CC-BY 40.

Figure 9

Schematic view of nanointerfaces probed by different XAS detection techniques. In this example, phase A (black) is a carbon-based nanoparticle and phase B (blue) an aqueous electrolyte. The element specificity is illustrated by showing two different excitation energies where A or B are absorbing the soft X-rays. The probing (yellow) and information (burgundy) volumes are indicated.

Figure 10

XAS of the nanodiamond-water interfacial region. XAS of nanodiamond aqueous dispersion at the carbon K-edge in fluorescence yield (a) and at the oxygen K-edge in transmission (b). Clear changes of the surface chemistry are observed at the carbon K-edge compared to dry nanodiamond while water reorganization is visible for increasing nanodiamond concentration at the oxygen K-edge. Panel a adapted with permission from ref (98). Copyright 2015 Royal Society of Chemistry. Panel b adapted with permission from ref (7). Copyright 2015 American Chemical Society.

Figure 11

Interfacial reactions monitored on nanoparticle dispersions by XAS in the hard X-ray range. (a, b) Protonation of SiO₂ nanoparticles is evidenced at the Si K-edge (a) and difference spectra between pH 0.3 and 10 (b) are found to depend on the nanoparticle size. (c) The Ce L3 XAS of ceria nanoparticles in water and cell culture medium (CCM) during the decomposition of H₂O₂ are shown. Panels a and b adapted from ref (104). Copyright 2012 American Chemical Society. Panel c adapted from ref (105). Copyright 2013 American Chemical Society.

Figure 12

Electron and ionic yield XAS at solid–water interface. (a) Electron yield XAS at the carbon K-edge of graphene in water before and after applied potential. (b) Electron yield XAS at the O K-edge of water on a gold film at different applied potentials. (c) Comparison of XAS measured in different detection modes at the C K-edge of carbon dots with amorphous (aCD), graphitic (gCD) and nitrogen-doped graphitic cores (N-gCD) in water. Panel a adapted from ref (106). Copyright 2013 IOP Publishing. Panels b adapted from ref (107). Copyright 2014 American Association for the Advancement of Science. Panel c adapted from ref (109). Copyright 2019 American Chemical Society.