The nature and spectroscopic expression of external surface sites of zeolites, in particular ZSM-5, is a long-debated question. Herein, we use three cutting-edge techniques: Fouriertransform infrared spectroscopy (FTIR) with Fourier selfdeconvolution (FSD), high magnetic field proton NMR spectroscopy under fast magic-angle spinning (MAS) and periodic boundary Density Functional Theory (DFT) calculations to study external surface models and analyze the effect of crystallite size. This provides an unequaled description of the various kinds of hydroxyl groups and of their proximities. The hydrogen-bond donor, acceptor or isolated nature of the hydroxyls results in distinct signals both in FTIR and NMR spectra, but the peak assignment is not the same from one technique to the other when the chemical nature of the hydroxyl changes. Bridging Si−(OH)− Al groups and Al−(H 2 O) lead to overlapping signals in one-dimensional 1 H MAS NMR, whereas their contributions are strongly different in FTIR spectra. However, quantification and proximity assessment could only be obtained by 1 H MAS NMR. With DFT, we confirm previous assignments for silanols and Si−(OH)−Al bridging OH groups. Other signals (between 3750 and 3600 cm −1 , and between 1 and 4 ppm) are not only assigned to extra-framework species (which we confirm with dedicated models), but also enclose the signature of sites exposed at the external surface of ZSM-5. In particular, Al−(H 2 O) species (∼3665 cm −1 ; 3.8, 2.6 ppm) and silanol−Al (∼3740, 3720, 3665 cm −1 ; 2.6, 2.2 ppm) contribute to several features depending on their environment. μ 1 -Al−OH are also present at the external surface in low amount, with a 3780 cm −1 signal in IR, and weak signals in the 0−2 ppm interval in 1 H MAS NMR.

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.